driving xl_watchdog() in order to avoid races accessing if_timer.
While at it relax the watchdog a bit by reloading it in xl_txeof()/
xl_txeof_90xB() if there are still packets enqueued.
- Use bus_get_dma_tag() so xl(4) works on platforms requiring it.
- Don't bother to set if_mtu to ETHERMTU, ether_ifattach() does that.