powerpc: Fix logic error in fixup_irqs

When SPARSE_IRQ is set, irq_to_desc() can
return NULL. While the code here has a
check for NULL, it's not really correct.
Fix it by separating the check for it.

This fixes CPU hot unplug for me.
Reported-by: Alastair Bridgewater <alastair.bridgewater@gmail.com>
Cc: stable@kernel.org [2.6.32+]
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/irq.c

@@ -295,7 +295,10 @@ void fixup_irqs(const struct cpumask *map)
 
 	for_each_irq(irq) {
 		desc = irq_to_desc(irq);
-		if (desc && desc->status & IRQ_PER_CPU)
+		if (!desc)
+			continue;
+
+		if (desc->status & IRQ_PER_CPU)
 			continue;
 
 		cpumask_and(mask, desc->affinity, map);