irqchip: spear_shirq: Use the proper interfaces

No point in doing a full irq lookup, when the desc pointer is
available.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20140619212713.404243909@linutronix.de


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