USB: f_fs: Add memory barrier before atomic operations

In few instances, it is observed that multiple adbd instances are
running on device causing condition BUG_ON(ffs->gadget) to be true.
ffs->opened and ffs->ref atomic variables are used here to make
decision for checking ffs->gadget. These atomic variable operations
requires expilict memory barrier to make sure that update to
ffs->gadget is visible to other CPUs before updated atomic variable
based value is seen.

CRs-Fixed: 793733
Change-Id: I3c846eb6bbb53663892e05d51ebac8439aac957a
Signed-off-by: Mayank Rana <mrana@codeaurora.org>