net: phy: at803x: move specific at8031 probe mode check to dedicated probe

Move specific at8031 probe mode check to dedicated probe to make
at803x_probe more generic and keep code tidy.

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