mmc: dw_mmc: use the cookie's enum values for post/pre_req()

This patch removed the meaningless value. Instead, use the cookie's enum
values for executing correctly.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>