[media] v4l2-dv-timings.h: fix polarity for 4k formats

The VSync polarity was negative instead of positive for the 4k CEA formats.
I probably copy-and-pasted these from the DMT 4k format, which does have a
negative VSync polarity.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reported-by: Martin Bugge <marbugge@cisco.com>
Cc: <stable@vger.kernel.org>      # for v4.1 and up
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>