net: phy: aquantia: Enable autoneg when on USXGMII

If System Interface protocol is USXGMII then enable USXGMII autoneg

Signed-off-by: Valentin Catalin Neacsu <valentin-catalin.neacsu@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>