[media] sh_vou: avoid going past arrays

Smatch reports two issues:
	drivers/media/platform/sh_vou.c:670 vou_adjust_output() error: buffer overflow 'vou_scale_v_num' 3 <= 4
	drivers/media/platform/sh_vou.c:670 vou_adjust_output() error: buffer overflow 'vou_scale_v_den' 3 <= 4

It seems that there's actually a bug here: the same var (idx) is used
as an index for vertical and horizontal scaling arrays. However,
there are 4 elements on the h arrays, and only 3 at the v ones.

On the first loop, it may select index 4 for the horizontal array.

In this case, if the second loop fails to select an index, the
code would keep using 4 for the vertical array, with is past of
the array sizes.

The intent here seems to use index 0, if the scale is not found.

So, use a separate var for the vertical index.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>