quectel.js: Add method to determine the modem dbus path

Occasionally the modem will not come up as mode `0` in the modemManager list, this is problematic because we currently hardcode the dbus modem path here: https://github.com/balena-io-playground/gps-tracker/blob/master/gps/src/libs/quectel.js#L10 which means dbus fails to connect to the modem in this case.