[IA64] Fix wrong assumption in acpi_gsi_to_irq

The ia64's acpi_gsi_to_irq() function assumes irq == vector. But in
fact irq can be different from vector. This patch fix this wrong
assumption.

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