drivers: net: phy: aquantia: drop XGMII as a valid system interface proto

Use either USXGMII or XFI in aquantia_set_proto and drop XGMII as a valid
protocol configuration.  The PHY doesn't support it, it's just used as an
alias for one of the other two protocols.

Signed-off-by: Florin Chiculita <florinlaurentiu.chiculita@nxp.com>
Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>