ACPI: bus: make acpi_bus_type const

Now that the driver core can properly handle constant struct bus_type,
move the acpi_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: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/bus.c

@@ -1097,7 +1097,7 @@ static void acpi_device_remove(struct device *dev)
 	put_device(dev);
 }
 
-struct bus_type acpi_bus_type = {
+const struct bus_type acpi_bus_type = {
 	.name		= "acpi",
 	.match		= acpi_bus_match,
 	.probe		= acpi_device_probe,

include/acpi/acpi_bus.h

@@ -582,7 +582,7 @@ void acpi_initialize_hp_context(struct acpi_device *adev,
 				void (*uevent)(struct acpi_device *, u32));
 
 /* acpi_device.dev.bus == &acpi_bus_type */
-extern struct bus_type acpi_bus_type;
+extern const struct bus_type acpi_bus_type;
 
 int acpi_bus_for_each_dev(int (*fn)(struct device *, void *), void *data);
 int acpi_dev_for_each_child(struct acpi_device *adev,