mmc: mmci: fix error return code in mmci_probe()

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

Fixes: f9bb304c

 ("mmc: mmci: Add support for setting pad type via pinctrl")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>