mmc: block: Implement card_busy_detect() for busy detection

To complete a data write request we poll for the card's status register
by sending CMD13. The are other scenarios when this polling method are
needed, which is why we here moves this code to it's own function. No
functional change.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>