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" <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

include/linux/device.h

···
 	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