USABLE_RX_BD
hi_water = USABLE_RX_BD(rxr) / 4;
rxr->free_rx_bd = USABLE_RX_BD(rxr);
rxr->max_rx_bd = USABLE_RX_BD(rxr);
ifp->if_nmbclusters = sc->rx_ring_cnt * USABLE_RX_BD(&sc->rx_rings[0]);