depending on the NIC and isn't used at all with HomePNA links)
instead of if_slowtimo() for driving dc_watchdog() in order to
avoid races accessing if_timer.
- Use bus_get_dma_tag() so dc(4) works on platforms requiring it.
- Don't bother to set if_mtu to ETHERMTU, ether_ifattach() does that.
- Remove an alpha remnant in dc_softc.