[media] media-device: check before unregister if mdev was registered

Most media functions that unregister, check if the corresponding register
function succeed before. So these functions can safely be called even if a
registration was never made or the component as already been unregistered.

Add the same check to media_device_unregister() function for consistency.

This will also allow to split the media_device_register() function in an
initialization and registration functions without the need to change the
generic cleanup functions and error code paths for all the media drivers.

Suggested-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/media-device.c

@@ -577,6 +577,8 @@ EXPORT_SYMBOL_GPL(__media_device_register);
  * media_device_unregister - unregister a media device
  * @mdev:	The media device
  *
+ * It is safe to call this function on an unregistered
+ * (but initialised) media device.
  */
 void media_device_unregister(struct media_device *mdev)
 {
@@ -584,6 +586,10 @@ void media_device_unregister(struct media_device *mdev)
 	struct media_entity *next;
 	struct media_interface *intf, *tmp_intf;
 
+	/* Check if mdev was ever registered at all */
+	if (!media_devnode_is_registered(&mdev->devnode))
+		return;
+
 	/* Remove all entities from the media device */
 	list_for_each_entry_safe(entity, next, &mdev->entities, graph_obj.list)
 		media_device_unregister_entity(entity);