[POWERPC] spufs: Don't yield nosched context

Nosched context sould never be scheduled out, thus we must not
deactivate them in spu_yield ever.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>