mmc: tegra: properly disable card clock

The new code to do the clock rate setting externally to the SDMMC
module has a shortcut to not propagate changes with a 0 rate to
the CAR by simply bailing out. This breaks proper cutting of the
card clock. Fix it by directly calling the correct sdhci function.

Fixes: a8e326a9 "mmc: tegra: implement module external clock change"
Signed-off-by: Lucas Stach <dev@lynxeye.de>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>