USB: f_mtp: Avoid queuing of receive_file_work for 0 length

MTP Server can issue ioctl for receive_file_work even if
xfer_length is 0. Mainly due to small files which are read
from mtp_read() itself. As there is no need to queue work
followed by flush_work in that case, add a check for same
to avoid wasting few msecs for every small file transferred.

Change-Id: I7b5395cc771c6f1ef842e787210ec02262b2b44f
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>