driver core: make the bus_type in struct device_driver constant

The pointer to struct bus_type in struct device_driver should only be
pointing to something that can never change now that the driver core has
fixed up the previously writable fields.  So mark it as a constant
pointer to enforce this and move forward with the goal of moving
bus_type into read-only memory.

Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Link: https://lore.kernel.org/r/20230313182918.1312597-28-gregkh@linuxfoundation.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/device/driver.h

@@ -95,7 +95,7 @@ enum probe_type {
  */
 struct device_driver {
 	const char		*name;
-	struct bus_type		*bus;
+	const struct bus_type	*bus;
 
 	struct module		*owner;
 	const char		*mod_name;	/* used for built-in modules */