x86/MCE/AMD: Make kobj_type structure constant

Since

  ee6d3dd4 ("driver core: make kobj_type constant.")

the driver core allows the usage of const struct kobj_type.

Take advantage of this to constify the structure definition to prevent
modification at runtime.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230217-kobj_type-mce-amd-v1-1-40ef94816444@weissschuh.net