mmc: sdhci-s3c: Handle error from mmc_of_parse()

Since mmc_of_parse() may fail, let's deal with it and thus do proper
error handling.

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