alpha/irq: Use access helper irq_data_get_affinity_mask()

This is a preparatory patch for moving irq_data struct members.
Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Matt Turner <mattst88@gmail.com>
Link: http://lkml.kernel.org/r/20150713075508.821878421@linutronix.deSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/alpha/kernel/irq.c

@@ -59,7 +59,7 @@ int irq_select_affinity(unsigned int irq)
 		cpu = (cpu < (NR_CPUS-1) ? cpu + 1 : 0);
 	last_cpu = cpu;
 
-	cpumask_copy(data->affinity, cpumask_of(cpu));
+	cpumask_copy(irq_data_get_affinity_mask(data), cpumask_of(cpu));
 	chip->irq_set_affinity(data, cpumask_of(cpu), false);
 	return 0;
 }