[media] cx231xx: disable 417 support from the Conexant video grabber

The 417 support doesn't work. Until someone can dig into this driver to
figure out why it isn't working the 417 support is disabled.
Sometimes you can actually stream a bit, but very soon the whole machine
crashes, so something is seriously wrong.
For the record, this was not introduced by my recent changes to this driver,
it was broken before that.

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