powerpc/spufs: wait for stable spu status in spu_stopped()

If the spu is stopping (ie, the SPU_STATUS_RUNNING bit is still set),
re-read the register to get the final stopped value.

Signed-off-by: Luke Browning <lukebrowning@us.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>