stmmac: fix an error code in stmmac_ptp_register()

PTR_ERR(NULL) is success.  We have to preserve the error code earlier.

Fixes: 7086605 ("stmmac: fix error check when init ptp")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.c

@@ -185,8 +185,10 @@ int stmmac_ptp_register(struct stmmac_priv *priv)
 	priv->ptp_clock = ptp_clock_register(&priv->ptp_clock_ops,
 					     priv->device);
 	if (IS_ERR(priv->ptp_clock)) {
+		int ret = PTR_ERR(priv->ptp_clock);
+
 		priv->ptp_clock = NULL;
-		return PTR_ERR(priv->ptp_clock);
+		return ret;
 	}
 
 	spin_lock_init(&priv->ptp_lock);