an UART interface could get stuck when a new interrupt condition
arose while servicing a previous interrupt. Since an interrupt was
already pending, no new interrupt would be triggered.
Avoid infinite recursion by flushing the Rx FIFO and marking an
overrun condition when we could not move the data from the Rx
FIFO to the receive buffer in toto. Failure to flush the Rx FIFO
would leave the Rx ready condition pending.
Note that the SAB 82532 already did this due to the nature of the
chip.
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