powerpc: vio: make vio_bus_type const

Since commit d492cc2573a0 ("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 <[e-mail]>
Signed-off-by: "Ricardo B. Marliere" <[e-mail]>
Reviewed-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://msgid.link/[e-mail]

arch/powerpc/include/asm/vio.h

···
  */
 #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

···
 };
 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,
···
 };
 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,