plugin: explicitly request QMI probing

Plugins which may support QMI ports need to explicitly request QMI probing
in cdc-wdm devices. This should also avoid probing cdc-wdm ports when we know
that the plugin doesn't support them (e.g. with Ericsson MBM devices).

https://bugzilla.gnome.org/show_bug.cgi?id=696701

plugins/generic/mm-plugin-generic.c

@@ -84,6 +84,7 @@ mm_plugin_create (void)
                       MM_PLUGIN_ALLOWED_SUBSYSTEMS, subsystems,
                       MM_PLUGIN_ALLOWED_AT,         TRUE,
                       MM_PLUGIN_ALLOWED_QCDM,       TRUE,
+                      MM_PLUGIN_ALLOWED_QMI,        TRUE,
                       NULL));
 }