nvme.c: Check Firmware Update Granularity in fwdl

nvme.c: Check Firmware Update Granularity in fwdl

FWUG indicates the granularity and alignment requirement of the
firmware image being updated by the Firmware Image Download command

FWUG is reported in 4 KiB units
0h indicates that no information on granularity is provided
FFh indicates there is no restriction
so when this is 0 value, use 4 KiB split as a default

Do not override when xfer is given by user
-> Support for ignore fwug in (fwug>mdts) case

Reviewed-by: Keith Busch <keith.busch@gmail.com>
Reported-by: Minsik Jeon <hmi.jeon@samsung.com>
Signed-off-by: Steven Seungcheol Lee <sc108.lee@samsung.com>