net: phy: Use 'autoneg' flag from phydev

Use the 'autoneg' flag available in phydev when checking if
autoneg is in use.

The previous implementation was checking directly in the PHY
if autoneg was supported. Some PHYs will report that autoneg
is supported, even when it is disabled. Thus it is not possible
to use that bit to determine if autoneg is currently in use or
not.

Signed-off-by: Alexandre Messier <amessier@tycoint.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>