net: phy: use phy_modify_mmd_changed in genphy_c45_an_config_aneg

As can be seen from the usage of the return value, we should use
phy_modify_mmd_changed() here.

Fixes: 9a5dc8af

 ("net: phy: add genphy_c45_an_config_aneg")
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>