usb: mtu3: set otg_sel for u2port only if works as dual-role mode

When set otg_sel(SSUSB_U2_PORT_OTG_SEL) for u2port which supports
dual-role mode, the controller will automatically switch mode
between host and device according to IDDIG signal. But if the
u2port only supports device mode, and no IDDIG pin is provided,
setting otg_sel may cause failure of detection by host.
So set it only for dual-role mode.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>