We'll use +CPCMREG=1/0 to start/stop USB audio function, and we will also report the specific ttyUSB port to be used for audio in the Call interface.
The new logic is available in a new 'MMSharedSimtech' interface which is implemented both by the generic AT-based MMBroadbandModemSimtech modem, and by a new QMI-based new MMBroadbandModemQmiSimtech. Fixes https://gitlab.freedesktop.org/mobile-broadband/ModemManager/issues/112