USB: f_fs: Move ep completion out of stack

Allocating completion on the stack may lead to
invalid access when udc irq tries to complete
the request but interrupted completion returns
immediately. This happens because request is not
held to be dequeued anymore making the completion
invalid.

Move the completions in ffs data like it is for ep0.

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