fix memory leak in fixed btusb_close

If the waker is killed before it can replay outstanding URBs, these URBs
won't be freed or will be replayed at the next open.  This patch closes
the window by explicitely discarding outstanding URBs.

Signed-off-by: Oliver Neukum <oliver@neukum.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>