powerpc/8xx: Fix build breakage with sparse irq changes

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
diff --git a/arch/powerpc/platforms/8xx/m8xx_setup.c b/arch/powerpc/platforms/8xx/m8xx_setup.c
index 385acfc..242954c 100644
--- a/arch/powerpc/platforms/8xx/m8xx_setup.c
+++ b/arch/powerpc/platforms/8xx/m8xx_setup.c
@@ -222,7 +222,7 @@
 	int cascade_irq;
 
 	if ((cascade_irq = cpm_get_irq()) >= 0) {
-		struct irq_desc *cdesc = irq_desc + cascade_irq;
+		struct irq_desc *cdesc = irq_to_desc(cascade_irq);
 
 		generic_handle_irq(cascade_irq);
 		cdesc->chip->eoi(cascade_irq);