[media] vivid: Rename variable

r_y and g_u now also contain the H and V components on the HSV formats.
Rename the variables to reflect this.

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