media: s5p-jpeg: Split s5p_jpeg_parse_hdr()

This patch moves the subsampling value decoding read from the JPEG
header into its own function. This new function is called
s5p_jpeg_subsampling_decode() and returns true if it successfully
decodes the subsampling value, false otherwise.

Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Acked-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>