usb *hci: Always set ux_status before usbd_xfer_schedule_timeout.

Add an assert to usbd_xfer_schedule_timeout to enforce this.

Since access to ux_status is serialized by the bus lock, and nothing
releases the bus lock in the interim, this doesn't make a functional
change.  But it does reduce confusion by readers, who no longer have
to worry if some callers got the order wrong.  It could also now
potentially be factored out in a subsequent commit.

(riastradh)