[POWERPC] cell: remove unused struct spu variable

Remove the mostly unused variable isrc from struct spu and a forgotten
function declaration.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>