plugin: avoid segfault when port driver is unknown

Based on a patch from Bastiaan Jacques <bastiaan@bjacques.org>

https://bugzilla.redhat.com/show_bug.cgi?id=1177799
https://bugs.freedesktop.org/show_bug.cgi?id=88864

src/mm-device.c

@@ -271,6 +271,7 @@ mm_device_utils_get_port_driver (GUdevDevice *udev_port)
     if (!driver && strncmp (name, "rfcomm", 6) == 0)
         driver = "bluetooth";
 
+    /* Note: may return NULL! */
     return driver;
 }