irqchip: sun4i: Staticize sun4i_irq_ack()

sun4i_irq_ack() is only referenced in this file, so make it static.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: http://lkml.kernel.org/r/1373010070.14756.2.camel@phoenix


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>