api,introspection: report list of drivers, not just one

Different ports of the same modem may get handled by different drivers. We
therefore need to provide a list of drivers (new `Modem.Drivers' property with
signature 'as') instead of just one (removed `Modem.Driver' property with
signature 's').

$ sudo mmcli -m 0 | grep drivers
           |        drivers: 'qcserial, qmi_wwan'

plugins/generic/mm-plugin-generic.c

@@ -42,14 +42,14 @@ int mm_plugin_minor_version = MM_PLUGIN_MINOR_VERSION;
 static MMBaseModem *
 create_modem (MMPlugin *self,
               const gchar *sysfs_path,
-              const gchar *driver,
+              const gchar **drivers,
               guint16 vendor,
               guint16 product,
               GList *probes,
               GError **error)
 {
     return MM_BASE_MODEM (mm_broadband_modem_new (sysfs_path,
-                                                  driver,
+                                                  drivers,
                                                   mm_plugin_get_name (self),
                                                   vendor,
                                                   product));