mcb: constify the struct device_type usage

Since commit aed65af1

 ("drivers: make device_type const"), the driver
core can properly handle constant struct device_type. Move the
mcb_carrier_device_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>
Signed-off-by: "Ricardo B. Marliere" <ricardo@marliere.net>
Link: https://lore.kernel.org/r/20240219-device_cleanup-mcb-v1-1-dc930e7dc11c@marliere.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>