[IA64] Remove compiler warinings about uninitialized variable in irq_ia64.c

This patch removes the following compiler warning messages.

  CC      arch/ia64/kernel/irq_ia64.o
arch/ia64/kernel/irq_ia64.c: In function 'create_irq':
arch/ia64/kernel/irq_ia64.c:343: warning: 'domain.bits[0u]' may be used uninitialized in this function
arch/ia64/kernel/irq_ia64.c: In function 'assign_irq_vector':
arch/ia64/kernel/irq_ia64.c:203: warning: 'domain.bits[0u]' may be used uninitialized in this function

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