net: phy: at803x: replace AT803X_DEVICE_ADDR with MDIO_MMD_PCS

Replace AT803X_DEVICE_ADDR with MDIO_MMD_PCS defined in mdio.h.

Signed-off-by: Luo Jie <luoj@codeaurora.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>