mmc: sdhci-s3c: Use devm_clk_get()

Use devm_clk_get() rather than clk_get() to make cleanup paths
more simple.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>