[POWERPC] Fix comment in kernel/irq.c

kernel/irq.c contains a comment that speaks of -1 and -2 as interrupt
numbers, but this is actually dependent on configuration options now.
Replace by NO_IRQ and NO_IRQ_ENABLED.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/irq.c

@@ -281,10 +281,10 @@ void do_IRQ(struct pt_regs *regs)
 
 	/*
 	 * Every platform is required to implement ppc_md.get_irq.
-	 * This function will either return an irq number or -1 to
+	 * This function will either return an irq number or NO_IRQ to
 	 * indicate there are no more pending.
-	 * The value -2 is for buggy hardware and means that this IRQ
-	 * has already been handled. -- Tom
+	 * The value NO_IRQ_IGNORE is for buggy hardware and means that this
+	 * IRQ has already been handled. -- Tom
 	 */
 	irq = ppc_md.get_irq();