USB net drivers, ethtool bus_info

This patch makes the USB network drivers that have ethtool support
expose only those stable IDs ... no more references to unstable values
like dev->bus->busnum (changes depending on order of modprobe)
or dev->devnum (assigned not quite randomly during enumeration :).

This supports more intelligent policies for user mode tools, such
as "always assign this address to the device on that port", as well
as making troubleshooting more practical when there are multiple
devices (of a given type) present ... you can tell which is which.