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>

drivers/mmc/host/dw_mmc.h

@@ -244,6 +244,7 @@ struct dw_mci_tuning_data {
  * @prepare_command: handle CMD register extensions.
  * @set_ios: handle bus specific extensions.
  * @parse_dt: parse implementation specific device tree properties.
+ * @execute_tuning: implementation specific tuning procedure.
  *
  * Provide controller implementation specific extensions. The usage of this
  * data structure is fully optional and usage of each member in this structure