powerpc: pmac: make macio_bus_type const

Since commit d492cc25 ("driver core: device.h: make struct
bus_type a const *"), the driver core can properly handle constant
struct bus_type, move the macio_bus_type variable to be a constant
structure as well, placing it into read-only memory which can not be
modified at runtime.
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: "Ricardo B. Marliere" <ricardo@marliere.net>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240212-bus_cleanup-powerpc2-v2-4-8441b3f77827@marliere.net

arch/powerpc/include/asm/macio.h

@@ -6,7 +6,7 @@
 #include <linux/of.h>
 #include <linux/platform_device.h>
 
-extern struct bus_type macio_bus_type;
+extern const struct bus_type macio_bus_type;
 
 /* MacIO device driver is defined later */
 struct macio_driver;

drivers/macintosh/macio_asic.c

@@ -136,7 +136,7 @@ static int macio_device_modalias(const struct device *dev, struct kobj_uevent_en
 
 extern const struct attribute_group *macio_dev_groups[];
 
-struct bus_type macio_bus_type = {
+const struct bus_type macio_bus_type = {
        .name	= "macio",
        .match	= macio_bus_match,
        .uevent	= macio_device_modalias,