USB: android: Fix mutex issue if ffs_close runs after ADB disable

While switching from ADB to non-ADB composition, adbd is also
stopped. Since, stop adbd happens asynchronously, there is a
possibility that closed_callback gets called after ADB is
disabled. In that scenario, closed_callback wouldn't be able to
use android_dev->mutex and may race with ffs_enable resulting in
different issues/crashes.

Change-Id: I7941bbea72139b22eb0ec785c29e9bdb76b6f6a3
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>