[media] v4l2-ctrls: replace is_volatile with V4L2_CTRL_FLAG_VOLATILE

With the new flag there is no need anymore to have a separate is_volatile
field. Modify all users to use the new flag.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>