So here's the EHCI implementation of that new callback,
morphed/simplified from the old "free_config" one (which
is now gone).  It looks almost identical to the OHCI
version, except the dummy TDs work a bit differently.

Again, drivers that clean themselves up in disconnect()
shouldn't notice this change.  I didn't re-test this;
I don't have devices with the other kind of driver.  You
should do so with one of yours (high speed hub and TT).

There are still about half a dozen places in usbcore
and the HCDs that would benefit from using this new
callback, FWIW.  I'd call them non-critical bugfixes
that should wait a bit, while this batch shakes out.