[media] v4l: vsp1: Reuse local variable instead of recomputing it

No need to waste CPU cycles when the value we need is already available.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>