[CELL] spufs: fix read and write for decr_status file

The decr_status in the LSCSA is valid only in the sequence of context
restore. Thus, it's nonsense to read and/or write it through spufs.

This patch changes decr_status node to access MFC_CNTL[Ds] in the CSA.

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>