[media] au0828: postpone call to au0828_unregister_media_device()

The DVB core needs to unregister the media device. So, we
can't call au0828_unregister_media_device() before calling
au0828_dvb_unregister(), otherwise the DVB MC free code
(that will be implemented on the next patch) will fail.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>