mmc: dw_mmc: Add missing description

Commit 0976f16d ("mmc: dw_mmc: add support tuning scheme") introduced
the execute_tuning hook but did not add its description for kernel docs.
Update the same.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <chris@printf.net>