usb: gadget: Resolve NULL pointer dereference in composite_disconnect

There is a race possibility in android_disconnect and
configfs_composite_unbind while using cdev leading to a NULL pointer
dereference in composite_disconnect. Combine android_disconnect with
configfs_composite_disconnect and remove the android_disconnect function.
configfs_composite_disconnect already has a gi->spinlock in place to
prevent the race condition.

Change-Id: Idfdebaf69f3aa68d90b55bffd7c2e04410c5a47f
Signed-off-by: Ronak Vijay Raheja <rraheja@codeaurora.org>