firmware: dmi-sysfs: make kobj_type structures constant

Since commit 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 definitions to prevent
modification at runtime.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20230227-kobj_type-firmware-dmi-v1-1-d1131d4e7a2b@weissschuh.net


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>