[media] cx25821: do not expose broken video output streams

The cx25821 driver has support for one audio output channel and two video
output channels.
This is implemented in a very ugly and very evil way through a custom ioctl
that passes the filename of a file containing the video data, which is then
read by the driver itself using vfs.
There are a number of problems with this:
1) it's very ugly and very evil (I can't say that often enough).
2) V4L2 supports video output, so why not use that?
3) it's very buggy, closing the filehandle through which you passed the ioctl
   will oops the kernel.
4) it's a nasty security leak since this allows you to load any file in the
   system as a video or audio source, so in theory you can output /etc/passwd
   to audio or video out and record & decode it on another device.
Because of all these issues we no longer register those output video nodes.

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