V4L/DVB (8920): cx18/ivtv: fix check of window boundaries for VIDIOC_S_FMT

It was possible to set out-of-bounds windows sizes, this is now
fixed.

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