platform/surface: aggregator: make ssam_bus_type constant and static

Now that the driver core can properly handle constant struct bus_type,
move the ssam_bus_type variable to be a constant structure as well,
placing it into read-only memory which can not be modified at runtime.

It's also never used outside of
drivers/platform/surface/aggregator/bus.c so make it static and don't
export it as no one is using it.

Cc: Maximilian Luz <luzmaximilian@gmail.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Cc:  <platform-driver-x86@vger.kernel.org>
Reviewed-by: Maximilian Luz <luzmaximilian@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/2023121957-tapered-upswing-8326@gregkhSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/platform/surface/aggregator/bus.c

@@ -35,6 +35,8 @@ static struct attribute *ssam_device_attrs[] = {
 };
 ATTRIBUTE_GROUPS(ssam_device);
 
+static const struct bus_type ssam_bus_type;
+
 static int ssam_device_uevent(const struct device *dev, struct kobj_uevent_env *env)
 {
 	const struct ssam_device *sdev = to_ssam_device(dev);
@@ -329,13 +331,12 @@ static void ssam_bus_remove(struct device *dev)
 		sdrv->remove(to_ssam_device(dev));
 }
 
-struct bus_type ssam_bus_type = {
+static const struct bus_type ssam_bus_type = {
 	.name   = "surface_aggregator",
 	.match  = ssam_bus_match,
 	.probe  = ssam_bus_probe,
 	.remove = ssam_bus_remove,
 };
-EXPORT_SYMBOL_GPL(ssam_bus_type);
 
 /**
  * __ssam_device_driver_register() - Register a SSAM client device driver.

include/linux/surface_aggregator/device.h

@@ -193,7 +193,6 @@ struct ssam_device_driver {
 
 #ifdef CONFIG_SURFACE_AGGREGATOR_BUS
 
-extern struct bus_type ssam_bus_type;
 extern const struct device_type ssam_device_type;
 
 /**