mmc: sdhci: don't use card state polling when CD GPIO is defined

There is no reason to use polling for card detection state change when
drivers are using dedicated GPIO for this. Don't poll in this case.

Signed-off-by: Ivan T. Ivanov <ivan.ivanov@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>