perf ftrace latency: Fix unit on histogram first entry when using --use-nsec

perf ftrace latency: Fix unit on histogram first entry when using --use-nsec

The use_nsec arg wasn't being taken into account when printing the first
histogram entry, fix it:

  root@number:~# perf ftrace latency --use-nsec -T switch_mm_irqs_off -a sleep 2
  #   DURATION     |      COUNT | GRAPH                                          |
       0 - 1    us |          0 |                                                |
       1 - 2    ns |          0 |                                                |
       2 - 4    ns |          0 |                                                |
       4 - 8    ns |          0 |                                                |
       8 - 16   ns |          0 |                                                |
      16 - 32   ns |          0 |                                                |
      32 - 64   ns |        125 |                                                |
      64 - 128  ns |        335 |                                                |
     128 - 256  ns |       2155 | ####                                           |
     256 - 512  ns |       9996 | ###################                            |
     512 - 1024 ns |       4958 | #########                                      |
       1 - 2    us |       4636 | #########                                      |
       2 - 4    us |       1053 | ##                                             |
       4 - 8    us |         15 |                                                |
       8 - 16   us |          1 |                                                |
      16 - 32   us |          0 |                                                |
      32 - 64   us |          0 |                                                |
      64 - 128  us |          0 |                                                |
     128 - 256  us |          0 |                                                |
     256 - 512  us |          0 |                                                |
     512 - 1024 us |          0 |                                                |
       1 - ...  ms |          0 |                                                |
  root@number:~#

After:

  root@number:~# perf ftrace latency --use-nsec -T switch_mm_irqs_off -a sleep 2
  #   DURATION     |      COUNT | GRAPH                                          |
       0 - 1    ns |          0 |                                                |
       1 - 2    ns |          0 |                                                |
       2 - 4    ns |          0 |                                                |
       4 - 8    ns |          0 |                                                |
       8 - 16   ns |          0 |                                                |
      16 - 32   ns |          0 |                                                |
      32 - 64   ns |         19 |                                                |
      64 - 128  ns |         94 |                                                |
     128 - 256  ns |       2191 | ####                                           |
     256 - 512  ns |       9719 | ####################                           |
     512 - 1024 ns |       5330 | ###########                                    |
       1 - 2    us |       4104 | ########                                       |
       2 - 4    us |        807 | #                                              |
       4 - 8    us |          9 |                                                |
       8 - 16   us |          0 |                                                |
      16 - 32   us |          0 |                                                |
      32 - 64   us |          0 |                                                |
      64 - 128  us |          0 |                                                |
     128 - 256  us |          0 |                                                |
     256 - 512  us |          0 |                                                |
     512 - 1024 us |          0 |                                                |
       1 - ...  ms |          0 |                                                |
  root@number:~#

Fixes: 84005bb6148618cc ("perf ftrace latency: Add -n/--use-nsec option")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Gabriele Monaco <gmonaco@redhat.com>
Link: https://lore.kernel.org/r/ZyE3frB-hMXHCnMO@x1
Signed-off-by: Namhyung Kim <namhyung@kernel.org>