mtdinfo: fix `--all' for non-consecutive device numbers

mtdinfo: fix `--all' for non-consecutive device numbers

When we have assigned non-consecutive device numbers to our MTD devices,
then we run `mtdinfo --all', we get errors once mtdinfo tries to process
the devices in the "hole". For instance, suppose that at boot time, we have
one MTD (/dev/mtd0) then perform a sequence like the following:

 # modprobe mtdram
 # modprobe nandsim
 # rmmod mtdram

Then at this point, we have should have devices 0 and 2 without 1. Then:

 # mtdinfo --all
 ...
 mtdinfo: error!: mtd1 does not correspond to any existing MTD device

We add a check to first see if device is present, then continue to the next
ID if it doesn't exist.

Reported-by: Brian Foster <brian.foster@maxim-ic.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>