genirq: Make function __irq_do_set_handler() static

Fixes the following sparse warning:

kernel/irq/chip.c:786:1: warning:
 symbol '__irq_do_set_handler' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: http://lkml.kernel.org/r/1474817799-18676-1-git-send-email-weiyj.lk@gmail.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/irq/chip.c

@@ -782,7 +782,7 @@ void handle_percpu_devid_irq(struct irq_desc *desc)
 		chip->irq_eoi(&desc->irq_data);
 }
 
-void
+static void
 __irq_do_set_handler(struct irq_desc *desc, irq_flow_handler_t handle,
 		     int is_chained, const char *name)
 {