plugins: generic: Add 'wwan' subsystem support

wwan is a new subsystem for WWAN devices, allowing to expose the
WWAN device and its ports in a generic way.

The sysfs hierarchy for such device is

/sys/class/wwanX
/sys/class/wwanX/wwanXpYP

Where X is the WWAN device index, Y the port index and P the
control protocol name (QMI, MBIM...). A control port is also
exposed as character device in /dev.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>

plugins/generic/mm-plugin-generic.c

@@ -91,7 +91,7 @@ create_modem (MMPlugin *self,
 G_MODULE_EXPORT MMPlugin *
 mm_plugin_create (void)
 {
-    static const gchar *subsystems[] = { "tty", "net", "usbmisc", NULL };
+    static const gchar *subsystems[] = { "tty", "net", "usbmisc", "wwan", NULL };
 
     return MM_PLUGIN (
         g_object_new (MM_TYPE_PLUGIN_GENERIC,