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>

drivers/macintosh/rack-meter.c

@@ -427,7 +427,7 @@ static int rackmeter_probe(struct macio_dev* mdev,
 	rm->irq = macio_irq(mdev, 1);
 #else
 	rm->irq = irq_of_parse_and_map(i2s, 1);
-	if (rm->irq == NO_IRQ ||
+	if (!rm->irq ||
 	    of_address_to_resource(i2s, 0, &ri2s) ||
 	    of_address_to_resource(i2s, 1, &rdma)) {
 		printk(KERN_ERR