xen: pcpu: make xen_pcpu_subsys const

Now that the driver core can properly handle constant struct bus_type,
move the xen_pcpu_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/20240203-bus_cleanup-xen-v1-1-c2f5fe89ed95@marliere.netSigned-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/pcpu.c

@@ -65,7 +65,7 @@ struct pcpu {
 	uint32_t flags;
 };
 
-static struct bus_type xen_pcpu_subsys = {
+static const struct bus_type xen_pcpu_subsys = {
 	.name = "xen_cpu",
 	.dev_name = "xen_cpu",
 };