[POWERPC] spufs: Fix memory leak on SPU affinity

Reference count for the "neighbor" spu context was not
being correctly decremented after usage.
So, contexts used as reference during SPU affinity setup
were not being deallocated, leading to a memory leak.

Signed-off-by: Andre Detsch <adetsch@br.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>