Clear sub_peer state on hangup to avoid "486 BUSY" in next call
Scenario:
- EXT: make a call from ext to DUT
- DUT: answer a call
- DUT: flash hook to hold a call from ext
- DUT: onhook
- DUT: starts ringing as ext initiate held call
- EXT: cancel call (DUT did not answer)
Result: Not possible to make any incomming or outgoing call from DUT.