driver core: device.h: make struct bus_type a const *

Now that all users who accessed the bus_type structure in struct device
are properly using it as a const *, mark it as such so that no one can
modify it going forward anymore.

Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Link: https://lore.kernel.org/r/20230313182918.1312597-35-gregkh@linuxfoundation.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/device.h

@@ -566,7 +566,7 @@ struct device {
 	const char		*init_name; /* initial name of the device */
 	const struct device_type *type;
 
-	struct bus_type	*bus;		/* type of bus device is on */
+	const struct bus_type	*bus;	/* type of bus device is on */
 	struct device_driver *driver;	/* which driver has allocated this
 					   device */
 	void		*platform_data;	/* Platform specific data, device