[POWERPC] spufs: save MFC command channel before purging MFC queue

During the context save process, we currently save the MFC command
channel after purging the MFC queues. This causes a systemsim warning,
as the command channel may be in an unknown state after the purge.

This change does the save before purging the MFC queues.

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