But as you illustrated, there is a large number of drivers that already 
assume no IRQ is < 0, even if they don't use any IRQ #0 themselves.  
That is a much bigger problem to fix.

And a much larger number assuming the reverse is true which are hiding
potential bugs on ARM.

Looking at the serial stuff the best checks appear to be looking at
"irq", "-1" and NO_IRQ.

For migration stuff that's doing broken things like

	if (irq < 0)

can be changed to

	if (irq <= 0)

and that can be done before NO_IRQ itself is nailed on ARM and PA-RISC.