V4L/DVB (10851): cx18: Fix a video scaling check problem introduced by sliced VBI changes

Fix a scaling check that was failing, due to a magic number I missed fixing
during previous slice VBI changes.  Now

  $ v4l2-ctl -v width=480,height=480,pixelformat=MPEG

yields proper visual results again.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>