backlight: constify backlight_ops

backlight_device_register has been expecting a const "ops" argument, and using
it as such, since 9905a43b2d563e6f89e4c63c4278ada03f2ebb14. Let's make the
remaining backlight_ops instances const.

Inspired by hunks of the grsecurity patch, updated for newer kernels.

Signed-off-by: Lionel Debroux <lionel_debroux@yahoo.fr>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/drivers/platform/x86/thinkpad_acpi.c b/drivers/platform/x86/thinkpad_acpi.c
index 304333f..efda9c5 100644
--- a/drivers/platform/x86/thinkpad_acpi.c
+++ b/drivers/platform/x86/thinkpad_acpi.c
@@ -6109,7 +6109,7 @@
 			       BACKLIGHT_UPDATE_HOTKEY);
 }
 
-static struct backlight_ops ibm_backlight_data = {
+static const struct backlight_ops ibm_backlight_data = {
 	.get_brightness = brightness_get,
 	.update_status  = brightness_update_status,
 };