mmc: mediatek: fix CRC error when calling mmc_select_hs400()

the tune result of hs200 mode at 200Mhz is not suitable for 50Mhz,
mmc_select_hs400() will set clock frequency to 50Mhz, use defalut
tune setting for 50Mhz to avoid CRC error.

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