powerpc: vio: make vio_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 vio_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-v2-2-8441b3f77827@marliere.net

arch/powerpc/include/asm/vio.h

@@ -39,7 +39,7 @@
  */
 #define VIO_CMO_MIN_ENT 1562624
 
-extern struct bus_type vio_bus_type;
+extern const struct bus_type vio_bus_type;
 
 struct iommu_table;
 

arch/powerpc/platforms/pseries/vio.c

@@ -1615,7 +1615,7 @@ static struct attribute *vio_cmo_dev_attrs[] = {
 };
 ATTRIBUTE_GROUPS(vio_cmo_dev);
 
-struct bus_type vio_bus_type = {
+const struct bus_type vio_bus_type = {
 	.name = "vio",
 	.dev_groups = vio_cmo_dev_groups,
 	.bus_groups = vio_bus_groups,
@@ -1634,7 +1634,7 @@ static struct attribute *vio_dev_attrs[] = {
 };
 ATTRIBUTE_GROUPS(vio_dev);
 
-struct bus_type vio_bus_type = {
+const struct bus_type vio_bus_type = {
 	.name = "vio",
 	.dev_groups = vio_dev_groups,
 	.uevent = vio_hotplug,