LastFrag
rx_ring[i].length = cpu_to_le32(PKT_BUF_SZ | LastFrag);
tx_ring[1].length = cpu_to_le32(LastFrag | PKT_BUF_SZ);
rx_ring[entry].length = cpu_to_le32(PKT_BUF_SZ | LastFrag);
tx_ring[0].length = cpu_to_le32(s | LastFrag);