phy: amlogic: phy-meson-gxl-usb2: default to host mode

The USB2 PHY can switch between PHY_MODE_USB_HOST and
PHY_MODE_USB_DEVICE. However, it cannot do it on it's own since it
requires re-routing of the corresponding USB pins from dwc3 (which is
used for host-mode) to dwc2 (which is used for device-mode).
Thus we don't need to auto-detect the mode based on the USB controller,
which simplifies the driver code.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>