[media] DocBook/demux.xml: Remove generic errors

Remove generic errors from ioctl() descriptions. For other ioctl's,
there's no generic section. So, just keep whatever is there.

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