V4L/DVB (6686): saa7134: fix composite over s-video input on the Tevion MD 9717

The vmux for composite over s-video input was wrong.

Signed-off-by: Hermann Pitton <hermann-pitton@arcor.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>