[media] tm6000: Disable video interface in radio mode

Video data is useless in radio mode, so the corresponding interface can
be safely disabled. This should reduce the amount of isochronous traffic
noticeably.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>