Streamline stack/l2cap/l2c_link::l2c_link_process_num_completed_pkts

Towards readable code

Bug: 163134718
Tag: #refactor
Test: act.py -tc BleCocTest
Test: ble paired 2 phones
Test: classic paired Bose SoundLink

Change-Id: I1d6072e5f526c7f2ac1b3229c2e4bdbcdbf3d083