phylink: avoid attaching more than one PHY

Attaching more than one PHY to phylink is bad news, as we store a
pointer to the PHY in a single location. Error out if more than one
PHY is attempted to be attached.
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/phylink.c

@@ -725,6 +725,9 @@ int phylink_connect_phy(struct phylink *pl, struct phy_device *phy)
 		     phy_interface_mode_is_8023z(pl->link_interface))))
 		return -EINVAL;
 
+	if (pl->phydev)
+		return -EBUSY;
+
 	/* Use PHY device/driver interface */
 	if (pl->link_interface == PHY_INTERFACE_MODE_NA) {
 		pl->link_interface = phy->interface;