net: phy: swphy: emulate register MII_ESTATUS

When the genphy driver binds to a swphy it will call
genphy_read_abilites that will try to read MII_ESTATUS if BMSR_ESTATEN
is set in MII_BMSR. So far this would read the default value 0xffff
and 1000FD and 1000HD are reported as supported just by chance.
Better add explicit support for emulating MII_ESTATUS.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>