[POWERPC] spufs: No need to have a runnable SPU for libassist update

We don't need to update the libassist statistic with the context in a
runnable state, so do it after spu_disable_spu().

Signed-off-by: Luke Browning <lukebrowning@us.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>