net: phy: at803x: remove probe and struct at803x_priv

struct at803x_priv is never used in this driver. So remove it
and the probe function allocating it.

Suggested-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>