Add DTMF support for QMI modems
QMI modems use native QMI calls for call management, but not yet for DTMF. For this feature, a fallback to AT commands is used, but some modems do not have an AT port.
This MR implements the native QMI calls for DTMF.
Depends on libqmi!327 (merged)
Fixes #543 (closed)
Edited by Dylan Van Assche