mmc: dw_mmc: exynos: fix the NULL pointer dereference error

"host->cur_slot" should be assigned to start the request.
So it can be the NULL pointer. This patch fixed this error.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>