s3cmci: attach get_cd host ops

Attach the routine to get_cd to allow the MMC core to find out whether
there is a card present or not without the tedious process of trying to
send commands to the card or not.

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