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>
diff --git a/src/include/device/device.h b/src/include/device/device.h
index 48a5bcc..744836d 100644
--- a/src/include/device/device.h
+++ b/src/include/device/device.h
@@ -185,7 +185,7 @@
 void assign_resources(struct bus *bus);
 const char *dev_name(struct device *dev);
 const char *dev_path(const struct device *dev);
-u32 dev_path_encode(struct device *dev);
+u32 dev_path_encode(const struct device *dev);
 const char *bus_path(struct bus *bus);
 void dev_set_enabled(struct device *dev, int enable);
 void disable_children(struct bus *bus);