mmc: mxcmmc: fix error return code in mxcmci_probe()

platform_get_irq() returns an error code, but the mxcmmc driver
ignores it and always returns -EINVAL. This is not correct,
and prevents -EPROBE_DEFER from being propagated properly.

Print error message and propagate the return value of
platform_get_irq on failure.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>