mg_disk: fix error return code in mg_probe()

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

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>