sunxi: musb: Stop treating not having a vbus-det gpio as an error

On some boards the otg is wired up in host-only mode in this case we
have no vbus-det gpio.

Stop logging an error from sunxi_usb_phy_vbus_detect() in this case, and
stop treating sunxi_usb_phy_vbus_detect() returning a negative errno, as
if a charger is plugged into the otg port.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Ian Campbell <ijc@hellion.org.uk>