[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>