mmc: mediatek: drop too much code of tuning method

the tuning code is becoming more and more bloated, let's make the
set cmd/data delay to inline function to avoid too much redundant code.

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