usb: android: Fix possible race in terms of using ffs_function

When adbd is being killed and restarted, android_disable() and
android_enable() APIs are called from functionfs_closed_callback()
and functionfs_ready_callback(). config->func is being allocated
and updated with mutex_lock acquire whereas it is freed without
mutex_lock from functionfs_closed_callback(). Hence make sure
to allocate and de-allocate ffs_function with mutex_lock acquire.

CRs-Fixed: 888043
Change-Id: I015afe81e5e93694a462be609a119a7fdae2c928
Signed-off-by: Mayank Rana <mrana@codeaurora.org>