net/macb: fix error return code in macb_probe()

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

Original-idea-by: <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>