I noticed a minor goof, basically an open issue coming home to
roost.  For the root hub, usb_device->devpath was "/" so it
caused problems using devpath when constructing names for
interfaces.  Driverfs doesn't reject it, but of course the
resulting directory names can't be used...

This patch (untested, but compiles) should make devpath in that
case be "0" ... which can't collide with any path through hub
ports (first port == "1") so it's possible to use driverfs
paths again.