net: mvneta: mark as a legacy_pre_march2020 driver

mvneta provides mac_an_restart and mac_pcs_get_state methods, so needs
to be marked as a legacy driver. Marek spotted that mvneta had stopped
working in 2500base-X mode - thanks for reporting.

Reported-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>