powerpc/pseries: Fix xics interrupt affinity

Commit 57b150cc (irq: only update affinity if
->set_affinity() is sucessfull) broke xics irq affinity.

We need to use the cpumask passed in, instead of accessing ->affinity directly.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>