media: mediatek: vcodec: decoder: Const-ify stepwise_fhd

stepwise_fhd is the reference framesize variable, and should not be
altered. Make it constant.

Fixes: ("76250b48 media: mediatek: vcodec: Getting supported decoder format types")
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>