xhci(4): Fix handling of endpoint reset/stop.

Use the same asynchronous task resetting a stalled/halted endpoint
and stopping a running endpoint -- either way we need to put the
endpoint back into a known state and, if there are transfers waiting
to run, start them up again.

- xhci_abortx must not drop the pipe (bus) lock -- usbdi(9) requires
  this.  So arrange to stop the endpoint and empty the queue
  asynchronously.

- If the xhci softint claims an xfer for completion with
  usbd_xfer_trycomplete, it must call usb_transfer_complete without
  ever releasing the pipe (bus) lock -- it can't claim the xfer and
  then defer the usb_transfer_complete to a task.  So arrange to
  reset the endpoint asynchronously, hold up new transfers until the
  endpoint has been reset, and then do usb_transfer_complete
  immediately.

(riastradh)