firmware: coreboot: make coreboot_bus_type const

Now that the driver core can properly handle constant struct bus_type,
move the coreboot_bus_type 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/20240204-bus_cleanup-firmware-v1-1-d1bff946606d@marliere.net


Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>