net: phy: at803x: move specific qca808x config_aneg to dedicated function

Move specific qca808x config_aneg to dedicated function to permit easier
split of qca808x portion from at803x driver.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>