[media] s5p-jpeg: avoid overwriting JPEG_CNTL register settings

Take into account the JPEG_CNTL register value read before
setting SYS_INT_EN bit field.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>