net: phy: at803x: add MDIX support to AR8031/33

AR8035 recently gained MDIX support. The same functions will work for
the AR8031/33 PHY. We just need to add the at803x_config_aneg()
callback.

This was tested on a Kontron sl28 board.
Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/at803x.c

@@ -1190,6 +1190,7 @@ static struct phy_driver at803x_driver[] = {
 	.probe			= at803x_probe,
 	.remove			= at803x_remove,
 	.config_init		= at803x_config_init,
+	.config_aneg		= at803x_config_aneg,
 	.soft_reset		= genphy_soft_reset,
 	.set_wol		= at803x_set_wol,
 	.get_wol		= at803x_get_wol,