mmc: sunxi: Gate the clock when rate is 0

The MMC core assumes that the code will gate the clock when the bus
frequency is set to 0, which we've been ignoring so far.

Handle that.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Tested-by: Florian Vaussard <florian.vaussard@heig-vd.ch>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>