enetc: Fix inconsistent IS_ERR and PTR_ERR

The proper pointer to be passed as argument is hw
Detected using Coccinelle.

Fixes: 6517798d ("enetc: Make MDIO accessors more generic and export to include/linux/fsl")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>