s3cmci: ensure host stopped on machine shutdown

Ensure that the s3cmci host controller is turned off
when the machine is shutdown, otherwise we end up
leaving the card powered and processing insertion and
removal events after the system prints "System halted."

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>