mmc: return mmc_of_parse() errors to caller

In addition to just logging errors encountered during DT parsing or
allocating GPIO slots for CD/WP, mmc_of_parse() now returns with an error.

In particular, this is needed if the GPIO allocation may return
EPROBE_DEFER.

Signed-off-by: Simon Baatz <gmbnomis@gmail.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>