net: phy: report link partner features through ethtool

The PHY library already reads the MII_STAT1000 and MII_LPA registers in
genphy_read_status(), so extend it to also populate the PHY device link
partner advertised features such that we can feed this back into ethtool
when asked for it in phy_ethtool_gset().

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>