nubus: Make nubus_bus_type static and constant

Now that the driver core can properly handle constant struct bus_type,
move the nubus_bus_type variable to be a constant structure as well,
placing it into read-only memory which can not be modified at runtime.

It's also never used outside of drivers/nubus/bus.c so make it static
and don't export it as no one is using it.

Signed-off-by: Greg Kroah-Hartman <[e-mail]>
Acked-by: Finn Thain <[e-mail]>
Reviewed-by: Geert Uytterhoeven <[e-mail]>
Link: https://lore.kernel.org/r/2023121940-enlarged-editor-c9a8@gregkh
Signed-off-by: Geert Uytterhoeven <[e-mail]>

drivers/nubus/bus.c

···
 		ndrv->remove(to_nubus_board(dev));
 }
 
-struct bus_type nubus_bus_type = {
+static const struct bus_type nubus_bus_type = {
 	.name		= "nubus",
 	.probe		= nubus_device_probe,
 	.remove		= nubus_device_remove,
 };
-EXPORT_SYMBOL(nubus_bus_type);
 
 int nubus_driver_register(struct nubus_driver *ndrv)
 {

include/linux/nubus.h

···
 	void (*remove)(struct nubus_board *board);
 };
 
-extern struct bus_type nubus_bus_type;
-
 /* Generic NuBus interface functions, modelled after the PCI interface */
 #ifdef CONFIG_PROC_FS
 extern bool nubus_populate_procfs;