[IA64] Fix invalid irq vector assumption for iosapic

Many of IOSAPIC codes depends on the flollowing assumptions, but these
would become invalid when multiple vector domain will be supported in
the future.

  - 1:1 mapping between IRQ and vector
  - IRQ == vector

To fix those invalid assumptions, this patch changes iosapic_intr_info[]
to be indexed by irq number instead of 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>