x86: fix warning in arch/x86/kernel/io_apic.c

this warning:

  arch/x86/kernel/io_apic.c: In function ‘ir_set_msi_irq_affinity’:
  arch/x86/kernel/io_apic.c:3373: warning: ‘cfg’ may be used uninitialized in this function

triggers because the variable was truly uninitialized. We'd crash on
entering this code.

Signed-off-by: Ingo Molnar <mingo@elte.hu>