media: omap3isp: make omap3isp_prev_csc and omap3isp_prev_rgbtorgb const

Make these const as they are only used as a copy operation.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>