mmc: sdhci-brcmstb: handle mmc_of_parse() errors during probe

We need to handle mmc_of_parse() errors during probe otherwise the
MMC driver could start without proper initialization (e.g. power sequence).

Fixes: 476bf3d6 ("mmc: sdhci-brcmstb: Add driver for Broadcom BRCMSTB SoCs")
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>