x86/mce: Make mce_subsys const

Now that the driver core can properly handle constant struct bus_type,
make mce_subsys a constant structure.

Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20240204-bus_cleanup-x86-v1-1-4e7171be88e8@marliere.net

arch/x86/kernel/cpu/mce/core.c

@@ -2431,7 +2431,7 @@ static void mce_enable_ce(void *all)
 		__mcheck_cpu_init_timer();
 }
 
-static struct bus_type mce_subsys = {
+static const struct bus_type mce_subsys = {
 	.name		= "machinecheck",
 	.dev_name	= "machinecheck",
 };