Bluetooth: Host: Free ISO TX context

When disconnected while sending data, if ISO doesn't get the number of
completed packets it will not call `process_unack_tx` and thus will leak
TX context.

Fix that by setting the connection state in ISO disconnection which will
trigger a call to `process_unack_tx`.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>