net: ethernet: dwmac: Fix an error code in imx_dwmac_probe()

The code is return PTR_ERR(NULL) which is zero or success.  We should
return -ENOMEM instead.

Fixes: 94abdad6

 ("net: ethernet: dwmac: add ethernet glue logic for NXP imx8 chip")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>