net: phy: aquantia: add USXGMII support and warn if XGMII mode is set

So far we didn't support mode USXGMII, and in order to not break few
boards mode XGMII was accepted for the AQR107 family even though it
doesn't support XGMII. Add USXGMII support to the Aquantia PHY driver
and warn if XGMII mode is set.

v2:
- add warning if XGMII mode is set

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>