TXBUF2_EMPTY
if (sc->sc_txstate & TXBUF2_EMPTY) {
sc->sc_txstate &= ~(TXBUF2_EMPTY | TXBUF_MASK);
sc->sc_txstate |= TXBUF2_EMPTY;
sc->sc_txstate = TXBUF1_EMPTY | TXBUF2_EMPTY;