hci_usb.h: fix hard-to-trigger race

If someone tries to _urb_unlink while _urb_queue_head is running, he'll see
_urb->queue == NULL and fail to do any locking.  Prevent that from happening
by strategically placed barriers.

Signed-off-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>