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/ge/ge_pic.c

@@ -102,7 +102,7 @@ static void gef_pic_cascade(struct irq_desc *desc)
 	 */
 	cascade_irq = gef_pic_get_irq();
 
-	if (cascade_irq != NO_IRQ)
+	if (cascade_irq)
 		generic_handle_irq(cascade_irq);
 
 	chip->irq_eoi(&desc->irq_data);
@@ -206,7 +206,7 @@ void __init gef_pic_init(struct device_node *np)
 
 	/* Map controller */
 	gef_pic_cascade_irq = irq_of_parse_and_map(np, 0);
-	if (gef_pic_cascade_irq == NO_IRQ) {
+	if (!gef_pic_cascade_irq) {
 		printk(KERN_ERR "SBC610: failed to map cascade interrupt");
 		return;
 	}
@@ -223,12 +223,12 @@ void __init gef_pic_init(struct device_node *np)
 
 /*
  * This is called when we receive an interrupt with apparently comes from this
- * chip - check, returning the highest interrupt generated or return NO_IRQ
+ * chip - check, returning the highest interrupt generated or return 0.
  */
 unsigned int gef_pic_get_irq(void)
 {
 	u32 cause, mask, active;
-	unsigned int virq = NO_IRQ;
+	unsigned int virq = 0;
 	int hwirq;
 
 	cause = in_be32(gef_pic_irq_reg_base + GEF_PIC_INTR_STATUS);