mmc: sdhci-of-esdhc: poll ESDHC_FLUSH_ASYNC_FIFO bit until completion

The ESDHC_FLUSH_ASYNC_FIFO bit which is set to flush asynchronous FIFO
should be polled until it's auto cleared by hardware.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>