ion of the SPU thread.

This fixes a common problem of losing a mailbox write when it was done
to a saved context.

Signed-off-by: Andre Detsch <adetsch@br.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>
žh