Check for card detect each time an MMC/SD device is initialized. If card
detection is not implemented, this code behaves as before and continues
assuming a card is present. If no card is detected, has_init is reset
for the MMC/SD device (to force initialization next time) and an error
is returned.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Tested-by: Jason Liu <jason.hui@linaro.org>