mmc: Do not call get_cd for non removable cards

Non removable cards are always present, so do not call get_cd for them.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>