net: phy: Add detection of 1000BaseX link mode support

Add 1000BaseX to the link modes which are detected based on the
MII_ESTATUS register as per 802.3 Clause 22. This allows PHYs which
support 1000BaseX to work properly with drivers using phylink.

Previously 1000BaseX support was not detected, and if that was the only
mode the PHY indicated support for, phylink would refuse to attach it
due to the list of supported modes being empty.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>