mmc: sunxi: Add card busy detection

Some sdio wifi modules have not been working reliable with the sunxi-mmc
host code. This turns out to be caused by starting new io-rw commands while
the card signals that it is still busy processing a previous command.

This commit adds card-busy detection to the sunxi-mmc driver which together
with recent core changes to check card-busy before starting io-rw commands
fixes the wifi reliability issues on the Cubietruck and other sunxi boards
using sdio wifi.

Reported-by: Eugene K <sigintmailru@gmail.com>
Suggested-by: Eugene K <sigintmailru@gmail.com>
Cc: Eugene K <sigintmailru@gmail.com>
Cc: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>