mmc: dw_mmc: rockchip: Don't recalc the clock when it goes off

The "set_ios" function is called with a clock of 0 when the clock is
turning off.  There's no reason to go through all the extra Rockchip
logic (whose goal is to make sure DIV is 0 or 1) in that case.  The
Rockchip logic happened to work because the CCF will pick the lowest
possible rate when you ask it for a clock of 0, but it's silly to go
through all the remuxing and adjusting for no reason.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>