irqchip: spear_shirq: Kill the clear_reg nonsense

None of the chips has a ACK register. The code brainlessly fiddles
with the enable register, so it might even reenable a disabled
interrupt at least on spear300.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20140619212713.570396433@linutronix.de


Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>