fbdev: mmp: Constify struct mmp_overlay_ops

'struct mmp_overlay_ops' is not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  11798	    555	     16	  12369	   3051	drivers/video/fbdev/mmp/hw/mmp_ctrl.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  11834	    507	     16	  12357	   3045	drivers/video/fbdev/mmp/hw/mmp_ctrl.o
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Helge Deller <deller@gmx.de>

drivers/video/fbdev/mmp/hw/mmp_ctrl.c

@@ -313,7 +313,7 @@ static void path_set_mode(struct mmp_path *path, struct mmp_mode *mode)
 	mutex_unlock(&path->access_ok);
 }
 
-static struct mmp_overlay_ops mmphw_overlay_ops = {
+static const struct mmp_overlay_ops mmphw_overlay_ops = {
 	.set_fetch = overlay_set_fetch,
 	.set_onoff = overlay_set_onoff,
 	.set_win = overlay_set_win,

include/video/mmp_disp.h

@@ -156,7 +156,7 @@ struct mmp_overlay {
 	int status;
 	struct mutex access_ok;
 
-	struct mmp_overlay_ops *ops;
+	const struct mmp_overlay_ops *ops;
 };
 
 /* panel type */
@@ -299,7 +299,7 @@ struct mmp_path_info {
 	int overlay_num;
 	void (*set_mode)(struct mmp_path *path, struct mmp_mode *mode);
 	void (*set_onoff)(struct mmp_path *path, int status);
-	struct mmp_overlay_ops *overlay_ops;
+	const struct mmp_overlay_ops *overlay_ops;
 	void *plat_data;
 };