[media] videodev2.h: Extend struct v4l2_ext_controls

So it can be used to get the default value of a control.

Without this change it is not possible to get the
default value of array controls.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>