mmc: dw_mmc: improve the timeout polling code

Just use the readl_poll_timeout{_atomic} to avold open
coding them.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>