powerpc: Remove all usages of NO_IRQ

NO_IRQ has been == 0 on powerpc for just over ten years (since commit
0ebfff1491 ("[POWERPC] Add new interrupt mapping core and change
platforms to use it")). It's also 0 on most other arches.

Although it's fairly harmless, every now and then it causes confusion
when a driver is built on powerpc and another arch which doesn't define
NO_IRQ. There's at least 6 definitions of NO_IRQ in drivers/, at least
some of which are to work around that problem.

So we'd like to remove it. This is fairly trivial in the arch code, we
just convert:

    if (irq == NO_IRQ)	to	if (!irq)
    if (irq != NO_IRQ)	to	if (irq)
    irq = NO_IRQ;	to	irq = 0;
    return NO_IRQ;	to	return 0;

And a few other odd cases as well.

At least for now we keep the #define NO_IRQ, because there is driver
code that uses NO_IRQ and the fixes to remove those will go via other
trees.

Note we also change some occurrences in PPC sound drivers, drivers/ps3,
and drivers/macintosh.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/sysdev/tsi108_pci.c

@@ -433,7 +433,7 @@ void tsi108_irq_cascade(struct irq_desc *desc)
 	struct irq_chip *chip = irq_desc_get_chip(desc);
 	unsigned int cascade_irq = get_pci_source();
 
-	if (cascade_irq != NO_IRQ)
+	if (cascade_irq)
 		generic_handle_irq(cascade_irq);
 
 	chip->irq_eoi(&desc->irq_data);