The Lan996x controller supports the QUSGMII mode, which is very similar
to QSGMII in the way it's configured and the autonegociation
capababilities it provides.

This commit adds support for that mode, treating it most of the time
like QSGMII, making sure that we do configure the PCS how we should.
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>