[POWERPC] spufs: remove stop_code from struct spu

This patch remove 'stop_code' -- discarded member of struct spu.
It is written at initialize and interrupt, but never read
in current implementation.

Signed-off-by: Masato Noguchi <Masato.Noguchi@jp.sony.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>