intel_th: 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
intel_th_source_device_type, intel_th_output_device_type,
intel_th_switch_device_type and intel_th_device_type variables to be
constant structures as well, placing it into read-only memory which can not
be modified at runtime.

Signed-off-by: "Ricardo B. Marliere" <ricardo@marliere.net>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240429130119.1518073-7-alexander.shishkin@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>