mmc: sdhci-sprd: clear the UHS-I modes read from registers

sprd's sd host controller supports SDR50/SDR104/DDR50 though, the UHS-I
mode used by the specific card can be selected via devicetree only.

Fixes: fb8bd90f

 ("mmc: sdhci-sprd: Add Spreadtrum's initial host controller")
Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
Signed-off-by: Chunyan Zhang <zhang.lyra@gmail.com>
Reviewed-by: Baolin Wang <baolin.wang@linaro.org>
Tested-by: Baolin Wang <baolin.wang@linaro.org>
Cc: stable@vger.kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>