mmc: block: Respect hw busy detection in card_busy_detect()

Currently for write request we don't trust the hw busy detection to be
fully handled by host, thus we also poll the card's status until we see
it's gets out of the busy state.

Still there are scenarios where it will a benefit to trust the hw busy
detection done by the host, since no additional polling is needed.
Let's prepare card_busy_detect() to be able to handle this.

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