powerpc/powermac: Use set_hard_smp_processor_id() instead of smp_hw_index

The hard_smp_processor_id functions are the appropriate interfaces for
managing physical CPU ids.

Signed-off-by: Nathan Lynch <ntl@pobox.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>