V4L/DVB: V4L2 Spec: Improve the VIDIOC_QUERY_DV_PRESET description

Make explicit what should happen when the input signal is missing,
unreliable or does not map to a supported preset.

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