[POWERPC] spu sched: ensure preempted threads are put back on the runqueue

To not lose a spu thread we need to make sure it always gets put back
on the runqueue.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>