mmc: tmio: stop clock when 0Hz is requested

Setting frequency to 0 is not enough, the clock explicitly has to be
disabled. Otherwise voltage switching (which needs SDCLK to be quiet)
fails for various cards.

Because we now do the 'new_clock == 0' check right at the beginning,
the indentation level of the rest of the code can be decreased a little.

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