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 <[e-mail]>
Signed-off-by: Helge Deller <[e-mail]>

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

···
 	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

···
 	int status;
 	struct mutex access_ok;
 
-	struct mmp_overlay_ops *ops;
+	const struct mmp_overlay_ops *ops;
 };
 
 /* panel type */
···
 	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;
 };