Serial: bug in 8250.c when handling PCI or other level triggers

[Alan Cox]

The receive_chars function is designed to handle the case where the port
is jammed full on by aborting after 256 characters in one IRQ.
Unfortunately the author of this code forgot that some systems are level
triggered. On these systems the IRQ simply gets invoked again and the
count loop just makes the problem take longer to clear.

In the non level case it appears that a jam could leave the IRQ jammed
forever and ignored - the remaining bytes never appearing.

As far as I can see the code would actually need to clear the interrupt
enable and then restore it on a timer event or similar in order to get
the intended effect.

Alan

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