AWI_TXD_NEXT
sc->sc_txdone = awi_read_4(sc, sc->sc_txdone + AWI_TXD_NEXT) &
awi_write_4(sc, sc->sc_txbase + AWI_TXD_NEXT, 0);
awi_write_4(sc, txd + AWI_TXD_NEXT, ntxd);