[media] s5p-mfc: Correct scratch buffer size of H.263 decoder

Driver complains about too small scratch buffer size. After adjusting
it according to vendor code, decoding works.

[mszyprow: moved the change to the header file]

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>