[CELL] spufs: change decrementer restore timing

The SPU decrementer should be restored after the LSCSA DMA has
completed.

Signed-off-by: Masato Noguchi <Masato.Noguchi@jp.sony.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>