usb: phy: 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
usb_phy_dev_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-v1-2-77423c4da262@marliere.netSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/phy/phy.c

@@ -699,7 +699,7 @@ int usb_add_phy(struct usb_phy *x, enum usb_phy_type type)
 }
 EXPORT_SYMBOL_GPL(usb_add_phy);
 
-static struct device_type usb_phy_dev_type = {
+static const struct device_type usb_phy_dev_type = {
 	.name = "usb_phy",
 	.uevent = usb_phy_uevent,
 };