Plug an xfer leak when detaching root hubs.

Plug an xfer leak when detaching root hubs.

This leak is similar to the public xfer leak #1 that was affecting
device interrupt pipes except that root hubs are rarely detached.

Note that this xfer is never associated to any TD and is just used
to indicate that some of the HC ports has changed status, so there
is no need to flag it as "done" before completing it.