[POWERPC] Fix xics set_affinity code

On a POWER6 machine running 2.6.23-rc8 I sometimes see the following error:

xics_set_affinity: No online cpus in the mask 00000000,00000000,00000000,00000001 for irq 20

In a desperate attempt to get a changelog entry in 2.6.23, I took a look
into it.

It turns out we are passing a real and not a virtual irq into
get_irq_server.  This works for the case where hwirq < NR_IRQS and we
set virq = hwirq.  In my case however hwirq = 590082 and we try and
access irq_desc[590082], slightly past the end at 512 entries.

Lucky we ship lots of memory with our machines.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>