WIP: qmi-device: add mux-id property if QRTR is supported
This allows us to abstract which rmnet_data interface this QmiDevice corresponds to. If QMI_DEVICE_MUX_ID_AUTOMATIC is used, then at device open time, we search for the lowest N such that rmnet_dataN is not an available interface, and assign mux_id N+1. If another mux_id is given, then the mux ID is considered static and we will assume this device corresponds to a client/data port that already exists.
This mux ID should be used in the WDS Bind Mux Data Port message.