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: NUlf Hansson <ulf.hansson@linaro.org>
Acked-by: NJaehoon Chung <jh80.chung@samsung.com>