mmc: dw_mmc: use the 'slot' instead of 'cur_slot'

Remove the 'cur_slot'. Instead, just use 'slot'.
There is no multiple slots, so we need to consider only one slot.

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>