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>