USB: android: Fix a NULL pointer dereference

Fix a possible NULL pointer dereference of dev pointer in
android_disable() called from adb_closed_callback().  It happens
when adb device file is opened/closed and adb is not in the
current composition.

CRs-Fixed: 492464
Change-Id: I80f646658ce56c6883eb79773705e01967f9234e
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>