V4L/DVB (6341): ivtv: fix resizing MPEG1 streams

Resizing an MPEG 1 stream would cut off the right half of the
image due to a missing divide by 2 in VIDIOC_S_FMT.

Also did some minor cleanup in this part of the code.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>