Do not pass an xfer pointer to the timeout routine checking for root hub

Do not pass an xfer pointer to the timeout routine checking for root hub
status changes because it might be freed when detaching the root uhub(4).

Also do not reschedule a timeout if the pipe is being aborted.

Finally do not add more code to retrieve the 'bInterval' value of the
root hub endpoint descriptor since this value is hardcoded in the uhci(4)
driver.