mmc: mediatek: do not tune data for HS400 mode

for hs400 mode, should only tune DS delay, should not
tune PAD_TUNE for data path.
if eMMC will run at hs400 mode, do not tune data while
call ops->execute_tuning().

Signed-off-by: Chaotian Jing <chaotian.jing@mediatek.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>