usb: gadget: 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 gadget_type
variable to be a constant structure as well, placing it into read-only
memory which can not be modified at runtime.

Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Link: https://lore.kernel.org/r/20240218-device_cleanup-usb-v1-3-77423c4da262@marliere.net


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