USB: gadget: mass_storage: Fix Null pointer access during disconnect

There is a chance that completion handler and ep disable race each other
and it might happen that completion handler gets called after driver_data
is set to NULL as part of function disable. This results in crash. Hence
add check in completion handler to check if driver_data is NULL or not
to fix the issue.

Change-Id: I3496811f52af79e8dccb701b2753a81f5d5a2340
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>