USB: f_fs: Cutoff epfile IO before epfile could get freed

epfile may get freed and accessing epfile's error flag to
cut off IOs may lead to use after free.

Move the epfile error flag setting above in the order
so that it guaranteed to be valid.

CRs-Fixed: 668046
Change-Id: I0017513393ddb4fd288cd4e1c2adf9d5ee3bc660
Signed-off-by: Sujeet Kumar <ksujeet@codeaurora.org>