mmc: sdhci: add get_cd() implementation

1. mmc_rescan will call get_cd to know whether the card is present
before mmc_rescan_try_freq to avoid useless trials during
card removal or start host is called when card is not present.

2. get_cd needs to be checked to resolve slow card removal issue.

Signed-off-by: Kevin Liu <kliu5@marvell.com>
Signed-off-by: Chris Ball <cjb@laptop.org>