ath79: ag71xx: defer probe if of_phy_connect failed

gmac0 may need a phy on builtin switch, which can be unavailable
if gmac0 is probed before builtin switch.
Return -EPROBE_DEFER in this case so that gmac0 can be probed
later.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>

target/linux/ath79/files/drivers/net/ethernet/atheros/ag71xx/ag71xx_phy.c

@@ -75,8 +75,8 @@ int ag71xx_phy_connect(struct ag71xx *ag)
 
 	if (!ag->phy_dev) {
 		dev_err(&ag->pdev->dev,
-			"Could not connect to PHY device\n");
-		return -ENODEV;
+			"Could not connect to PHY device. Deferring probe.\n");
+		return -EPROBE_DEFER;
 	}
 
 	dev_info(&ag->pdev->dev, "connected to PHY at %s [uid=%08x, driver=%s]\n",