net: phy: at803x: Use helpers to access MMD PHY registers

Libphy provides a standard set of helpers to access the MMD PHY
registers. Use those instead of relying on custom driver-specific
functions.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>