Bluetooth: controller: llcp: fix issue re. missing ack of terminate ind

On remote terminate on central the conn clean-up would happen before ack
of terminate ind was sent to peer.
Now clean-up is 'postponed' until subsequent event.
Also now data tx is paused on rx of terminate ind to ensure no data is
tx'ed after rx of terminate ind

Signed-off-by: Erik Brockhoff <erbr@oticon.com>