[POWERPC] spufs: fix post-stopped update of MFC_CNTL register

We currently have two issues with the MFC save code:

 * save_mfc_decr doesn't handle a transition of 1 -> 0 of the Ds bit
 * The Q bit may be stale in the CSA

This change fixes the first issue by clearing the relevant bits from
the MFC_CNTL value in the CSA before or-ing in the updated status.
Also, we add the Q bit to the updated status.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>