fp-device: Unref the usb device on finalize

Each device adds a ref to the underlying usb device, but it doesn't remove
the reference on finalization.

So clear the object to fix the leak

libfprint/fp-device.c

@@ -382,6 +382,7 @@ fp_device_finalize (GObject *object)
 
   g_clear_pointer (&priv->device_id, g_free);
   g_clear_pointer (&priv->device_name, g_free);
+  g_clear_object (&priv->usb_device);
 
   G_OBJECT_CLASS (fp_device_parent_class)->finalize (object);
 }