usb: gadget: Do not disconnect unregistered dev

configfs_composite_unbind sets the gadget data
to null. Therefore, add check in android_disconnect
function to make sure that cdev is not NULL. This
prevents NULL pointer dereference upon accessing
cdev.

Change-Id: Ib59cdd88557d917c85d8d33968efc295bcef2280
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>