usb: dwc3: Set default mode for dwc_usb31

dwc_usb31 does not support OTG mode. If the controller supports DRD but
the dr_mode is not specified or set to OTG, then set the mode to
peripheral.

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>