irqchip: sun4i: Simplify sun4i_irq_ack

Now that we only ack irq 0 the code can be simplified a lot.

Also switch from read / modify / write to a simple write clear:
1) This is what the android code does (it has a hack for acking irq 0
   in its unmask code doing this)

2) read / modify / write simply does not make sense for an irq status
   register like this, if the other bits are writeable (and the data sheet says
   they are not) they should be write 1 to clear, since otherwise a read /
   modify / write can race with a device raising an interrupt and then clear
   the pending bit unintentionally

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-sunxi@googlegroups.com
Link: http://lkml.kernel.org/r/1394895894-8891-3-git-send-email-hdegoede@redhat.com


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