src/device: Fix discarded-qualifiers compilation issue

coreboot build is broken due to
CL: I6830a65bc9cea2907f4209bb97a53ccebcbf248d

This patch ensures to build coreboot successfully.

Change-Id: I4c9dfc9b19ce159ce1abcfbb287be4ce273cbaf1
Signed-off-by: Subrata Banik <subrata.banik@intel.com>
Reviewed-on: https://review.coreboot.org/25985
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/device/device_util.c

@@ -209,7 +209,7 @@ struct device *dev_find_class(unsigned int class, struct device *from)
  * @param dev The device path to encode.
  * @return Device path encoded into lower 3 bytes of dword.
  */
-u32 dev_path_encode(struct device *dev)
+u32 dev_path_encode(const struct device *dev)
 {
 	u32 ret;