[media] dvbdev: returns error if graph object creation fails

Right now, if something gets wrong at dvb_create_media_entity()
or at dvb_create_media_graph(), the device will still be
registered.

Change the logic to properly handle it and free all media graph
objects if something goes wrong at dvb_register_device().

Also, change the logic at dvb_create_media_graph() to return
an error code if something goes wrong. It is up to the
caller to implement the right logic and to call
dvb_unregister_device() to unregister the already-created
objects.

While here, add a missing logic to unregister the created
interfaces.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>