[POWERPC] spufs: fix assignment of node numbers

The difference between 'nid' and 'node' fields in an
spu structure was used incorrectly. The common 'node'
number now reflects the NUMA node, and it is used
in other places in the code as well.

The 'nid' value is meaningful only in one place, namely
the computation of the interrupt numbers based on the
physical location of an spu.  Consequently, we look it
up directly in the place where it is used now.

Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>