V4L/DVB (9191): ivtv: partially revert an earlier patch that checks the max image...

Ian Armstrong pointed out to me that it is perfectly valid to have a
576 lines YUV video with NTSC output in this particular case. In fact,
without this the X driver for ivtv does not function properly.

Reverted my earlier change that restrictred the height to 480 for NTSC
and add the comments provided by Ian to explain what is happening here.

Thanks-to: Ian Armstrong <ian@iarmst.co.uk>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>