bus-independent backend, with PCI and CardBus attachment code.
The committed code has two serious bugs:
1. The driver makes no attempt to recover resources when a (Cardbus)
instance is removed; bus resources are leaked.
2. In testing with a NetGear GA-511, the Cardbus card never responded
to a reset/wakeup if the card is powered down after attachment.
So for now, leave cardbus instances powered up at attachment
(insertion, or at boot if a card is already present).
That aside, it acutally works on my GA-511. Committed as-is despite
the bugs, after repeated requests to make the code available for
further testing. Also requires sys/dev/mii/miidevs rev 1.54 -> 1.55,
and consequent regen of miidevs{,_data}.h.
