mmc: dw_mmc: reduce timeout detection cycle

It's no need to speed 10 seconds to wait the mmc device out from busy
status. 500 milliseconds enough.

Signed-off-by: Ziyuan Xu <xzy.xu@rock-chips.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Tested-by: Jaehoon Chung <jh80.chung@samsung.com>