s390/time: make stp_subsys const

Now that the driver core can properly handle constant struct bus_type,
move the stp_subsys 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/20240204-bus_cleanup-s390_time-v1-1-d2120156982a@marliere.netSigned-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/kernel/time.c

@@ -716,7 +716,7 @@ static void stp_work_fn(struct work_struct *work)
 /*
  * STP subsys sysfs interface functions
  */
-static struct bus_type stp_subsys = {
+static const struct bus_type stp_subsys = {
 	.name		= "stp",
 	.dev_name	= "stp",
 };