usb: phy: constify the struct device_type usage

usb: phy: constify the struct device_type usage

Since commit aed65af1cc2f ("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.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>