mmc: dw_mmc: restore the card-present checking point

Restore the card-present checking point.

(The following part was removed from commit bf626e55 ("mmc: dw_mmc:
use slot-gpio to handle cd pin")

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>