mmc: sdhci: Fix tuning reset after exhausting the maximum number of loops

If the driver has exhausted the maximum number of tuning loops, then fixed
sampling is used. To do that both SDHCI_CTRL_TUNED_CLK and
SDHCI_CTRL_EXEC_TUNING must be reset to 0, but only SDHCI_CTRL_TUNED_CLK
was being reset. Reset SDHCI_CTRL_EXEC_TUNING to 0 also.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>