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zs serial and interrupt sensitivity

Hello all,

I've never had particularly good luck with the built-in serial ports on my 
9500, and my inability to use my ImageWriter II on one port with getty on the 
other for a console was irritating me (getty would die constantly under normal 
use prompting init to issue messages, and the other port would then wedge up 
typically in the middle of a print job).

I looked into the zs.c driver, and the one thing that stood out to me as wrong 
was that the interrupts were being registered as edge sensitive.  My gut 
feeling is that it should be level-sensitive; I'm not as familiar with the 8530 
as I am with the 16550 (they're quite different beasts), but I've never seen a 
serial port done as anything other than a level-sensitive interrupt because 
that's a good way to lose characters (especially if one comes in while you're 
servicing an interrupt).

Switching the interrupts to level sensitive seems to have fixed my problems.  
Can anyone think of a good reason this should be edge-sensitive?

- Dave

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