V4L/DVB (6718): ivtv: ivtv yuv format description correction

The driver was incorrectly reporting that it supported YUV 4:2:2 output, when
it is actually YUV 4:2:0. Though I believe the hardware can be pushed to
4:2:2, we don't currently support that.

Signed-off-by: Ian Armstrong <ian@iarmst.demon.co.uk>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>