net: phy: at803x: use genphy_config_init()

Use the generic bits from genphy_config_init() instead of implementing
the same functionality again.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>