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