irq-metag*: stop set_affinity vectoring to offline cpus

Fix irq_set_affinity callbacks in the Meta IRQ chip drivers to AND
cpu_online_mask into the cpumask when picking a CPU to vector the
interrupt to.

As Thomas pointed out, the /proc/irq/$N/smp_affinity interface doesn't
filter out offline CPUs, so without this patch if you offline CPU0 and
set an IRQ affinity to 0x3 it vectors the interrupt onto CPU0 even
though it is offline.

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-metag@vger.kernel.org
Cc: stable@vger.kernel.org