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

drivers/usb/gadget/function/u_ether.c

@@ -718,7 +718,7 @@ static const struct net_device_ops eth_netdev_ops = {
 	.ndo_validate_addr	= eth_validate_addr,
 };
 
-static struct device_type gadget_type = {
+static const struct device_type gadget_type = {
 	.name	= "gadget",
 };