s390/cio: make css_bus_type const

Now that the driver core can properly handle constant struct bus_type,
move the css_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: "Ricardo B. Marliere" <ricardo@marliere.net>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20240203-bus_cleanup-s390-v1-2-ac891afc7282@marliere.netSigned-off-by: Heiko Carstens <hca@linux.ibm.com>

drivers/s390/cio/css.c

@@ -39,7 +39,7 @@ int max_ssid;
 
 #define MAX_CSS_IDX 0
 struct channel_subsystem *channel_subsystems[MAX_CSS_IDX + 1];
-static struct bus_type css_bus_type;
+static const struct bus_type css_bus_type;
 
 int
 for_each_subchannel(int(*fn)(struct subchannel_id, void *), void *data)
@@ -1409,7 +1409,7 @@ static int css_uevent(const struct device *dev, struct kobj_uevent_env *env)
 	return ret;
 }
 
-static struct bus_type css_bus_type = {
+static const struct bus_type css_bus_type = {
 	.name     = "css",
 	.match    = css_bus_match,
 	.probe    = css_probe,