drivers: net: phy: at803x: Add missing mdio device id

at803x supports Atheros 8030, 8031 and 8035 PHYs. 8031 was missing from
the mdio device id table.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>