Add return condition to stack/l2cap/l2c_ble::l2cble_conn_comp

Eventually not pass control here

Bug: 170396407
Tag: #refactor
Test: act.py -tc BleCocTest
Test: ble paired 2 phones
Test: classic paired Bose SoundLink
Change-Id: Idda6b2ad34499ce1a7ad0e7770cb973a9b5d44ec