phy: sun4i-usb: Use devm_gpiod_get_optional for optional GPIOs

Both data->id_det_gpio and data->vbus_det_gpio are optional, so use
devm_gpiod_get_optional for them.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>