[media] cx25821: the audio channel was registered as a video node

Skip the audio channel when registering the video nodes. This fixes a bug
where that incorrectly registered 'video' node was never unregistered.
Note: this bug only surfaces if the video output nodes are enabled again
after the previous patch disabled them.

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