net: stmmac: clean up stmmac_mac_select_pcs()

Since all platform providers of PCS now populate the select_pcs()
method, there is no need for the common code to look at
priv->hw->phylink_pcs, so remove it.
Reviewed-by: Romain Gantois <romain.gantois@bootlin.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1sHhoh-00FetT-3S@rmk-PC.armlinux.org.ukSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -957,7 +957,7 @@ static struct phylink_pcs *stmmac_mac_select_pcs(struct phylink_config *config,
 			return pcs;
 	}
 
-	return priv->hw->phylink_pcs;
+	return NULL;
 }
 
 static void stmmac_mac_config(struct phylink_config *config, unsigned int mode,