[media] cx18/ivtv: Remove usage of V4L2_BUF_TYPE_PRIVATE

V4L2_BUF_TYPE_PRIVATE was used in these driver for internal purposes.
It turned out though that it wasn't used at all, so it could be removed.
I know it was used in the past, but clearly later changes made this
obsolete.

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