mmc: sdhci-esdhc-imx: clear ESDHC_STD_TUNING_EN for manual tuning method

The bit ESDHC_STD_TUNING_EN may be configed by bootloader code if it
choose to use standard tuning method. So on linux side, if choose to
use manual tuning method, need to clear the bit ESDHC_STD_TUNING_EN,
remove the impact of bootloader code.

Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>