[media] v4l: There's no __unsigned

Correct a typo. v4l2_plane.m.userptr is unsigned long, not __unsigned long.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>