V4L/DVB (5181): Sliced VBI API no longer marked experimental.

The Sliced VBI API is no longer marked experimental. Introduced in 2.6.14
and with only a single modification in 2.6.19 I think we can consider this
API to be solid.

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