[media] v4l2-ctrls: Fix control enumeration for multiple subdevs with ctrl

v4l2-ctl and qv4l2 enumerate controls using V4L2_CTRL_FLAG_NEXT_CTRL,
falling back to the old method if the flag isn't supported.

The v4l2_subdev_queryctrl function will currently handle that flag, but
for the controls of the subdev only. This isn't right, it should refuse
this flag, otherwise v4l2-ctl will only see the controls of the first
subdev.

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