powerpc/mpc62xx_pic: Fix get_irq handling of NO_IRQ

If none of irq category bits were set mpc52xx_get_irq() would pass
NO_IRQ_IGNORE (-1) to irq_linear_revmap, which does an unsigned compare
and declares the interrupt above the linear map range.  It then punts
to irq_find_mapping, which performs a linear search of all irqs,
which will likely miss and only then return NO_IRQ.

If no status bit is set, then we should return NO_IRQ directly.
The interrupt should not be suppressed from spurious counting, in fact
that is the definition of supurious.
Signed-off-by: Milton Miller <miltonm@bga.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/52xx/mpc52xx_pic.c

@@ -486,7 +486,7 @@ void __init mpc52xx_init_irq(void)
 unsigned int mpc52xx_get_irq(void)
 {
 	u32 status;
-	int irq = NO_IRQ_IGNORE;
+	int irq;
 
 	status = in_be32(&intr->enc_status);
 	if (status & 0x00000400) {	/* critical */
@@ -509,6 +509,8 @@ unsigned int mpc52xx_get_irq(void)
 		} else {
 			irq |= (MPC52xx_IRQ_L1_PERP << MPC52xx_IRQ_L1_OFFSET);
 		}
+	} else {
+		return NO_IRQ;
 	}
 
 	return irq_linear_revmap(mpc52xx_irqhost, irq);