[media] em28xx: disable ioctl VIDIOC_S_PARM for VBI devices

VIDIOC_S_PARM doesn't make sense for VBI device nodes, because we don't support
selecting the number of read buffers to use.

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