mmc: mediatek: Fixed bug where clock frequency could be set wrong

This patch can fix two issues:

Issue 1:
In previous code, div may be overflow when setting clock frequency
as f_min. We can use DIV_ROUND_UP to fix this boundary related
issue.

Issue 2:
In previous code, we can not set the correct clock frequency when
div equals 0xff.

Signed-off-by: Yong Mao <yong.mao@mediatek.com>
Signed-off-by: Chaotian Jing <chaotian.jing@mediatek.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>