mmc: dw_mmc: Convert to use DEFINE_SHOW_ATTRIBUTE

Use the newly added macro to simply to the code.

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