[PATCH] powerpc numa: Consolidate handling of Power4 special case

Code to handle Power4's invalid node id (0xffff) is duplicated for cpu
and memory.  Better to handle this case in one place --
of_node_to_nid.  Overall behavior should be unchanged.

Signed-off-by: Nathan Lynch <nathanl@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>