USB: f_mass_storage: Enable nofua flag for write control

nofua flag allows user to control ignore syncing data on to
media during write SCSI command though host has set the FUA bit,
when nofua flag is set.

As part of prevent allow SCSI command, all the data at filesystem
layer is synced to media and this is taking longer time for slow
media(class 2, class 4) and hence usb is re-enumerating. Hence,
avoid syncing data to media if nofua is set.

Change-Id: I562f05d6c0b8046e8a23a4df85385efc23aa364b
Signed-off-by: Rajkumar Raghupathy <raghup@codeaurora.org>