[media] vidioc-dv-timings-cap.xml: explicitly state that pad and reserved should be zeroed

The DV_TIMINGS_CAP documentation didn't state clearly that the pad and
reserved fields should be zeroed by the application. For subdev pad can
be other values as well.

It also mistakenly said that only drivers would have to zero the reserved
field, that's not correct.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>