mmc: tmio: disable clock before changing it

Rcar2 & 3 docs state that for going to and coming from the 0xff setting,
the clock must first be disabled before the DIV bits are changed.
Instead of tracking this, let's just do this unconditionally.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>