[media] v4l2-subdev: rename cropcap to g_pixelaspect

The old cropcap video op is now only used to pass the pixelaspect
ratio, so rename it.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>