net: phy: add GBit master / slave error detection

Certain PHY's have issues when operating in GBit slave mode and can
be forced to master mode. Examples are RTL8211C, also the Micrel PHY
driver has a DT setting to force master mode.
If two such chips are link partners the autonegotiation will fail.
Standard defines a self-clearing on read, latched-high bit to
indicate this error. Check this bit to inform the user.

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