mmc: mtk-sd: assign plat->cfg.f_max with a correct value

Currently this driver does not set the value of plat->cfg.f_max, which
results in that MMC framework will always run at the lowest frequency.

This patch sets plat->cfg.f_max to the maximum allowed frequency, which
equals to the source clock frequency.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>