V4L/DVB (10626): ivtv: fix regression in get sliced vbi format

The new v4l2_subdev_call used s_fmt instead of g_fmt.

Thanks-to: Andy Walls <awalls@radix.net>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>