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/ipic.c

@@ -853,7 +853,7 @@ void ipic_clear_mcp_status(u32 mask)
 	ipic_write(primary_ipic->regs, IPIC_SERMR, mask);
 }
 
-/* Return an interrupt vector or NO_IRQ if no interrupt is pending. */
+/* Return an interrupt vector or 0 if no interrupt is pending. */
 unsigned int ipic_get_irq(void)
 {
 	int irq;
@@ -864,7 +864,7 @@ unsigned int ipic_get_irq(void)
 	irq = ipic_read(primary_ipic->regs, IPIC_SIVCR) & IPIC_SIVCR_VECTOR_MASK;
 
 	if (irq == 0)    /* 0 --> no irq is pending */
-		return NO_IRQ;
+		return 0;
 
 	return irq_linear_revmap(primary_ipic->irqhost, irq);
 }