mmc: mediatek: fill the actual clock for mmc debugfs

as the mmc core layer has the mmc->actual_clock, so fill it
and drop msdc_host->sclk.

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