[BLUETOOTH]: rfcomm tty BUG_ON() code fix

1) In tty.c the BUG_ON at line 115 will never be called, because the the
   before list_del_init in this same function.
	115          BUG_ON(!list_empty(&dev->list));
   So move the list_del_init to rfcomm_dev_del 

2) The rfcomm_dev_del could be called from diffrent path
   (rfcomm_tty_hangup/rfcomm_dev_state_change/rfcomm_release_dev),

   So add another BUG_ON when the rfcomm_dev_del is called more than
   one time.

Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>