phy: sun4i-usb: Refactor VBUS detection to match Linux

The Linux driver checks the VBUS detection GPIO first; then VBUS power
supply; then finally assumes VBUS is present. When adding VBUS power
supply support, we want to match that order, so we get the same behavior
in case both a GPIO and a power supply are provided in the device tree.

So refactor the function a bit to remove the early return, and use the
same "assume VBUS is present" final fallback.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Acked-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>