mmc: sdhci-esdhc-imx: correct the tuning-step setting

Here we use '|=' to set the tuning-step, but before that, we should
clear the tuning-step, otherwise we could got the wrong setting.

Signed-off-by: Haibo Chen <haibo.chen@freescale.com>
Acked-by: Dong Aisheng <aisheng.dong@freescale.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>