USB: f_fs: Avoid using completion variable on stack

done completion variable is local stack variable to ffs_epfile_io().
It is being used to unblock ffs_epfile_io() from USB request
completion context where done is accessed through req->context. If
ffs_epfile_io() is unblocked or interrupted due to epfile close or
any signal before USB request completion is handled, req->context is
having stale "done" reference causing invalid access. Fix this issue
by storing done completion reference with epfile structure instead of
having it on stack to have valid req->context in completion handler.

CRs-Fixed: 653761
Change-Id: I15102538d1b5bee14dfa3c7b3fa1f8e3f767cf71
Signed-off-by: Sujeet Kumar <ksujeet@codeaurora.org>
Signed-off-by: Mayank Rana <mrana@codeaurora.org>