phy-sun4i-usb: Only check vbus-det on power-on on boards with vbus-det

data->vbus_det is always 1 on boards without a (working) vbus-det, skip
the vbus_det test on such boards.

This fixes the sun4i usb phy code never turning on Vbus on such boards.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>