It seems that clearing the MCR_IE bit in the modem control register

does not reliably prevent the triggering of interrupts for all supported
configurations. Thus, the FIFO size probe could cause an interrupt,
which could lead to an interrupt storm in the shared interrupt case.

To prevent this, change ns8250_bus_probe() to use the overflow bit in
the line status register instead of the RX ready bit in the interrupt
identification register to detect whether the FIFO has filled up.
This allows us to clear all bits in the interrupt enable register during
the probe, which should prevent interrupts reliably.
Additionally, the detected FIFO size may be a bit more accurate, because
the overflow bit is only set when the FIFO did actually fill up, while
interrupts would trigger a bit early.

Reviewed and tested on a lot of hardware by:	marcel

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