net: phy: at803x: move specific at8031 config_init to dedicated function

Move specific at8031 config_init to dedicated function to make
at803x_config_init more generic and tidy things up.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>