[IA64] Fix wrong access to irq_desc[] in iosapic_register_intr().

In error path we must unlock irq_desc[irq].lock before we change
'irq'.

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>