mmc: dw_mmc: remove the loop about finding slots

dwmmc controller has used the only one slot.
It doesn't need to check the other slots.
Remove the loop about finding slots.

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