ASY_ISR_FIFOEN
switch (isr & (ASY_ISR_FIFOEN | ASY_ISR_FIFO64)) {
case ASY_ISR_FIFOEN: /* 16550A with working FIFOs */
case ASY_ISR_FIFOEN | ASY_ISR_FIFO64: /* 16750 with 64byte FIFOs */