V4L/DVB (5438): Fix ivtv yuv threshold handling

Modifies automatic mode selection for yuv playback. Behaviour is now that
source video with a vertical resolution below that of the currently set
broadcast mode will be treated as progressive. Video with a vertical
resolution greater or equal to the current broadcast mode (up to 576 lines)
will be treated as interlaced.

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>