[media] uvcvideo: add support for VIDIOC_QUERY_EXT_CTRL

Add support for the new VIDIOC_QUERY_EXT_CTRL ioctl. Since uvc doesn't use
the control framework, support for this ioctl isn't automatic.

This is makes v4l2-compliance happy as well.

[Added blank lines for improved readability]

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>