V4L/DVB (8637): v4l2: add v4l2_ctrl_query_menu_valid_items support function

v4l2_ctrl_query_menu_valid_items() makes it easy to handle control menus
that have a lot of invalid 'holes'. For example, many MPEG encoders only
support a limited subset of audio bitrates. In that case a driver can
specify an array listing the set of valid bitrates and pass that to
this function.

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