usb: dwc3: Fix incorrect type for utmi mode

The utmi mode is unsigned according the dt-bindings.
Fix sparse issue (-Wtypesign):

  drivers/usb/dwc3/dwc3-omap.c:391:50: warning: incorrect type in argument 3 (different signedness)
  drivers/usb/dwc3/dwc3-omap.c:391:50:    expected unsigned int [usertype] *out_value
  drivers/usb/dwc3/dwc3-omap.c:391:50:    got int *<noident>

Signed-off-by: Franck Demathieu <fdemathieu@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>