net: phy: use genphy_read_abilities in genphy driver

Currently the genphy driver populates phydev->supported like this:
First all possible feature bits are set, then genphy_config_init()
reads the available features from the chip and remove all unsupported
features from phydev->supported. This can be simplified by using
genphy_read_abilities().

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>