Wrong values reported in Modem::MaxActiveBearers

The MaxActiveBearers property exposes the maximum amount of data connections we may have in a modem, and we compute this value based on the amount of data ports detected.

The logic deciding whether a given port can be used as data is a bit wrong, though. Right now we're assuming that all net ports are valid data ports (even if the plugin implementation may not support net ports) and that all AT ports are also valid data ports (even if the plugin implementation would use only the available net ports).

We need a way to build a correct list of data ports in mm_base_modem_organize_ports(), e.g. with new modem object properties specifying whether the modem expects net ports to create custom bearers or whether the modem is ok using also TTY ports for data connection.

E.g. QMI modems with one single network interface should report MaxActiveBearers=1 even if they have valid AT ports, because the MMBroadbandModemQmi modem object always creates MMBearerQmi bearers objects.