mmc: sdhci-s3c: Check if clk_set_rate() succeeds

It is possible that we may fail to set the clock rate, if we do so then
log the failure and don't bother reprogramming the IP.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>