exynos: Fix passing of errors in exynos_mmc_init()

exynos: Fix passing of errors in exynos_mmc_init()

exynos_mmc_init() always returns zero, so for the caller
it looks like it never fails.

Correct this by returning the error code of process_nodes().
For process_nodes() do something similar and return early
when do_sdhci_init() fails.

v2: Only fail in process_nodes() if we fail on all
    available nodes.

Acked-by: Przemyslaw Marczak <p.marczak@samsung.com>
Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>