[IA64] Fix wrong assumption about irq and vector in msi_ia64.c

This patch fixes a wrong assumption in ia64 MSI code that IRQ equals
vector.

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