boot_device: Constify argument

Add const qualifier to first argument.

Change-Id: I6655e04401b6a7aa5cafb717ff6f46b80b96646e
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32703
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>

src/include/boot_device.h

@@ -65,7 +65,7 @@ int boot_device_rw_subregion(const struct region *sub,
  * by the region device.
  * Returns 0 on success, < 0 on error.
  */
-int boot_device_wp_region(struct region_device *rd,
+int boot_device_wp_region(const struct region_device *rd,
 				const enum bootdev_prot_type type);
 
 /*