USB: f_fs: Avoid NULL pointer dereference during epfile_io()

If composition switch happens while epfile_io() is in progress,
there is a chance that gadget pointer is accessed after unbind(),
which could be NULL. This could cause accessing NULL pointer and
results in panic. Fix the issue by accessing member variables from
local copy instead of directly accessing from ffs->gadget.

Change-Id: Iec61725da9c82e8c06e66acb8a6bdbc98ff9444a
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>