[media] s5p-jpeg: Perform fourcc downgrade only for Exynos4x12 SoCs

Change the driver variant check from "is not S5PC210"
to "is Exynos4" while checking whether YUV format needs
to be downgraded in order to prevent upsampling which
is not supported by Exynos4 SoCs family.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>