net: stmmac: fix NPD with phylink_set_pcs if there is no MDIO bus

priv->plat->mdio_bus_data is optional, some platforms may not set it,
however we proceed to look straight at priv->plat->mdio_bus_data->has_xpcs.

Since the xpcs is instantiated based on the has_xpcs property, we can
avoid looking at the priv->plat->mdio_bus_data structure altogether and
just check for the presence of the xpcs pointer.

Fixes: 11059740 ("net: pcs: xpcs: convert to phylink_pcs_ops")
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NVladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

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

@@ -1238,11 +1238,8 @@ static int stmmac_phy_setup(struct stmmac_priv *priv)
 	if (IS_ERR(phylink))
 		return PTR_ERR(phylink);
 
-	if (mdio_bus_data->has_xpcs) {
-		struct mdio_xpcs_args *xpcs = priv->hw->xpcs;
-
-		phylink_set_pcs(phylink, &xpcs->pcs);
-	}
+	if (priv->hw->xpcs)
+		phylink_set_pcs(phylink, &priv->hw->xpcs->pcs);
 
 	priv->phylink = phylink;
 	return 0;