net: stmmac: dwmac-intel: provide a select_pcs() implementation

Move the code returning the XPCS into dwmac-intel, which is the only
user of XPCS. Fill in the select_pcs() implementation only when we are
going to setup the XPCS, thus when it should be present.
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1sHhoR-00FetB-CP@rmk-PC.armlinux.org.ukSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/stmicro/stmmac/dwmac-intel.c

@@ -443,6 +443,16 @@ static void common_default_data(struct plat_stmmacenet_data *plat)
 	plat->rx_queues_cfg[0].pkt_route = 0x0;
 }
 
+static struct phylink_pcs *intel_mgbe_select_pcs(struct stmmac_priv *priv,
+						 phy_interface_t interface)
+{
+	/* plat->mdio_bus_data->has_xpcs has been set true, so there
+	 * should always be an XPCS. The original code would always
+	 * return this if present.
+	 */
+	return &priv->hw->xpcs->pcs;
+}
+
 static int intel_mgbe_common_data(struct pci_dev *pdev,
 				  struct plat_stmmacenet_data *plat)
 {
@@ -587,6 +597,7 @@ static int intel_mgbe_common_data(struct pci_dev *pdev,
 	    plat->phy_interface == PHY_INTERFACE_MODE_1000BASEX) {
 		plat->mdio_bus_data->has_xpcs = true;
 		plat->mdio_bus_data->default_an_inband = true;
+		plat->select_pcs = intel_mgbe_select_pcs;
 	}
 
 	/* Ensure mdio bus scan skips intel serdes and pcs-xpcs */

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

@@ -957,9 +957,6 @@ static struct phylink_pcs *stmmac_mac_select_pcs(struct phylink_config *config,
 			return pcs;
 	}
 
-	if (priv->hw->xpcs)
-		return &priv->hw->xpcs->pcs;
-
 	return priv->hw->phylink_pcs;
 }