[media] au0828: Unregister notifiers

If au0828 gets removed, we need to remove the notifiers.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>