mmc: sdhci-esdhc-imx: tuning bits should not be cleared during reset

We should not clear tuning bits during reset or the SD3.0/eMMC4.5 card
working on UHS mode may not work after reset since the former tuning
settings was lost.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>