Polling for the data-transfer window to become open is wasteful.  We
can eliminate the polling loop by using hw_step() as the handler for
an xfer_window_changed() event.

If the window is already open at the time of instantiation, then
xfer_window_changed() may never be called.  We can cover this case by
using hw_step() as the step() method of a one-shot process.  Since the
signature for an xfer_window_changed() method is identical to the
signature for a process step() method, the same function can be used
for both.

Signed-off-by: Michael Brown <mcb30@ipxe.org>