platform: Make platform_bus_type constant

Since commit d492cc2 ("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 <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Link: https://lore.kernel.org/r/20240823075544.144426-1-kunwu.chan@linux.dev
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: KunWuChan

drivers/base/platform.c

@@ -1474,7 +1474,7 @@ static const struct dev_pm_ops platform_dev_pm_ops = {
 	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

@@ -52,7 +52,7 @@ struct platform_device {
 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 *,