V4L/DVB (4050): Add NTSC sliced VBI support to the cx25840 module.

NTSC sliced VBI support by Christopher Neufeld <television@cneufeld.ca>
with additional fixes by Hans Verkuil <hverkuil@xs4all.nl>.

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