mmc: sdhci-sprd: Check the enable clock's return value correctly

Missed to check the enable clock's return value, fix it.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>