mmc: au1xmmc: fix error return code in au1xmmc_probe()

Fix to return a negative error code from the error handling
case instead of 0, as done elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>