platform: Make platform_bus_type constant

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 platform_bus_type variable to be a constant
structure as well, placing it into read-only memory which can not be
modified at runtime.

Cc: Greg Kroah-Hartman <[e-mail]>
Suggested-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Kunwu Chan <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/base/platform.c

···
 	USE_PLATFORM_PM_SLEEP_OPS
 };
 
-struct bus_type platform_bus_type = {
+const struct bus_type platform_bus_type = {
 	.name		= "platform",
 	.dev_groups	= platform_dev_groups,
 	.match		= platform_match,

include/linux/platform_device.h

···
 extern int platform_device_register(struct platform_device *);
 extern void platform_device_unregister(struct platform_device *);
 
-extern struct bus_type platform_bus_type;
+extern const struct bus_type platform_bus_type;
 extern struct device platform_bus;
 
 extern struct resource *platform_get_resource(struct platform_device *,