[media] s5p-jpeg: Prevent erroneous downscaling for Exynos3250 SoC

JPEG codec on Exynos3250 SoC produces broken raw image if a JPEG
image is decoded to YUV420 format and downscaled by a factor
greater than 2. Prevent this by asserting downscale ratio to 2.

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