mmc: sdhci-sprd: fixed incorrect clock divider

The register SDHCI_CLOCK_CONTROL should be cleared before config clock
divider, otherwise the frequency configured maybe lower than we
expected.

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>