Bluetooth: Host: ISO not clearing TX context on disconnect

[theob-pro]

** Describe the bug **

When disconnected while sending data, if ISO doesn't get the num 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.

** To reproduce **

Create an ISO channel that will send data, disconnect before receiving the HCI number of completed packets event.

The following BabbleSim test reproduce the issue: theob-pro@e2d8225