ag71xx: fix error return code in ag71xx_probe()

Fix to return error code -ENOMEM from the dmam_alloc_coherent() error
handling case instead of 0, as done elsewhere in this function.

Fixes: d51b6ce4 ("net: ethernet: add ag71xx driver")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>