teensy-3.x:Removed call to khci_usbattach

  The call is not need by the driver if CONFIG_USBDEV_BUSPOWERED=y
  The class register will enable the soft connect pull up. The
  khci_usbattach call only set a flag, and that  flag is only tested
  in the class register. On a USB powered device if we are running we
  are attached.