mmc: mmc: Use ->card_busy() to detect busy cards in __mmc_switch()

Some MMC hosts do not support MMC_CAP_WAIT_WHILE_BUSY, but implements the
->card_busy() callback. In such cases, extend __mmc_switch() to use this
method to check card status after switch command.

Signed-off-by: Chaotian Jing <chaotian.jing@mediatek.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>