s390/ccwgroup: make ccwgroup_bus_type const

Now that the driver core can properly handle constant struct bus_type,
move the ccwgroup_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-v1-1-ac891afc7282@marliere.netSigned-off-by: Heiko Carstens <hca@linux.ibm.com>

drivers/s390/cio/ccwgroup.c

@@ -31,7 +31,7 @@
  * to devices that use multiple subchannels.
  */
 
-static struct bus_type ccwgroup_bus_type;
+static const struct bus_type ccwgroup_bus_type;
 
 static void __ccwgroup_remove_symlinks(struct ccwgroup_device *gdev)
 {
@@ -465,7 +465,7 @@ static void ccwgroup_shutdown(struct device *dev)
 		gdrv->shutdown(gdev);
 }
 
-static struct bus_type ccwgroup_bus_type = {
+static const struct bus_type ccwgroup_bus_type = {
 	.name   = "ccwgroup",
 	.dev_groups = ccwgroup_dev_groups,
 	.remove = ccwgroup_remove,