[IA64] irq_ia64, use set_irq_chip

Don't access desc->chip directly, because them chip member will
disappear some time later.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Tony Luck <tony.luck@intel.com>

arch/ia64/kernel/irq_ia64.c

@@ -633,7 +633,7 @@ ia64_native_register_percpu_irq (ia64_vector vec, struct irqaction *action)
 	BUG_ON(bind_irq_vector(irq, vec, CPU_MASK_ALL));
 	desc = irq_desc + irq;
 	desc->status |= IRQ_PER_CPU;
-	desc->chip = &irq_type_ia64_lsapic;
+	set_irq_chip(irq, &irq_type_ia64_lsapic);
 	if (action)
 		setup_irq(irq, action);
 	set_irq_handler(irq, handle_percpu_irq);