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: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/io_apic.c

@@ -3360,7 +3360,7 @@ static void
 ir_set_msi_irq_affinity(unsigned int irq, const struct cpumask *mask)
 {
 	struct irq_desc *desc = irq_to_desc(irq);
-	struct irq_cfg *cfg;
+	struct irq_cfg *cfg = desc->chip_data;
 	unsigned int dest;
 	struct irte irte;