[IA64] Fix incorrect return value from ia64_setup_msi_irq()

Fix the problem that pci_enable_msi() fails on ia64 platform. The cause of
this problem is incorrect return value of ia64_setup_msi_irq(). It must
return 0 on success, instead of irq number.

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