V4L/DVB (4596): Fix saa7115 miscalculation that breaks NTSC

This repairs a problem introduced by a commit earlier applied.
Thanks-to: Hans Verkuil <hverkuil@xs4all.nl>

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>