Retry call hangup once if it failed (e.g. due to ongoing modem request)

Signed-off-by: Martti Piirainen <martti.piirainen@oss.tieto.com>

Ring-Call-Channel currently assumes that a "hangup" request to the modem always succeeds, and is followed by status indications. This can break if the modem is busy with another request, e.g. during a "answer waiting call and release held call" sequence.