mmc: sdhci-bcm-kona: Handle error from mmc_of_parse()

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

Cc: Christian Daudt <bcm@fixthebug.org>
Cc: Matt Porter <mporter@linaro.org>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>