USB: f_fs: Check error status before doing epfile I/O

Set error status before disabling endpoint during function
disable and also check error status before handling I/O. If error
status is set, return error status to read/write calls made by
userspace. Also set file's private data to NULL during epfile
release.

CRs-Fixed: 671880
Change-Id: I14b5ee541dfc18a7802ef4a8033878a7729d9adb
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>