powerpc/cpm2: Fix set interrupt type

This is a simple change to correct problems when using set_irq_type
on platforms using CPM2.  This code corrects the problem on most platform
but may have issues on 8272 derived platforms for some interrupts.

On 8272 PC2 & 3 are missing and PC 23 & 29 are added, which this patch
does not address.
Signed-off-by: NPaul Bilke <paul@conspiracy.net>
Reviewed-by: NAnton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: NKumar Gala <galak@kernel.crashing.org>

arch/powerpc/sysdev/cpm2_pic.c

@@ -165,7 +165,7 @@ static int cpm2_set_irq_type(unsigned int virq, unsigned int flow_type)
 			edibit = (14 - (src - CPM2_IRQ_EXT1));
 	else
 		if (src >= CPM2_IRQ_PORTC15 && src <= CPM2_IRQ_PORTC0)
-			edibit = (31 - (src - CPM2_IRQ_PORTC15));
+			edibit = (31 - (CPM2_IRQ_PORTC0 - src));
 		else
 			return (flow_type & IRQ_TYPE_LEVEL_LOW) ? 0 : -EINVAL;