metag: 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: James Hogan <james.hogan@imgtec.com>
Cc: metag <linux-metag@vger.kernel.org>
Signed-off-by: James Hogan <james.hogan@imgtec.com>