xtensa: Use irq_set_affinity instead of homebrewn code

There is no point in having an incomplete copy of irq_set_affinity()
for the hotplug irq migration code.

Use the core function instead and while at it switch to
for_each_active_irq()

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Chris Zankel <chris@zankel.net>
Cc: xtensa <linux-xtensa@linux-xtensa.org>
Link: http://lkml.kernel.org/r/20140223212736.664624945@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>