nvme-fc: ensure association_id is cleared regardless of a Disconnect LS

Code today only clears the association_id if a Disconnect LS is transmit.

Remove ambiguity and unconditionally clear the association_id if the
association has been terminated.

Signed-off-by: James Smart <jsmart2021@gmail.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>