net: ethernet: mtk-star-emac: fix error return code in mtk_star_enable()

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

Fixes: 8c7bd5a4 ("net: ethernet: mtk-star-emac: new driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Acked-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/1605180879-2573-1-git-send-email-zhangchangzhong@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>