mvneta: implement ethtool autonegotiation control

This patch allows to do
ethtool -s eth0 autoneg off
ethtool -s eth0 autoneg on
to disable or enable autonegotiation at run-time.
Without that functionality, the only way to control the autonegotiation
is to modify the device tree.

This is needed if you plan to use the same kernel with
different ethernet switches, the ones that support the in-band
status and the ones that not.

CC: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
CC: netdev@vger.kernel.org
CC: linux-kernel@vger.kernel.org

Signed-off-by: Stas Sergeev <stsp@users.sourceforge.net>
Signed-off-by: David S. Miller <davem@davemloft.net>