thunderbolt: 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
tb_domain_type, tb_retimer_type, tb_switch_type, usb4_port_device_type,
tb_service_type and tb_xdomain_type variables to be constant structures 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>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>