net: mtk_eth_soc: mark as a legacy_pre_march2020 driver

mtk_eth_soc has not been updated for commit 7cceb599 ("net: phylink:
avoid mac_config calls"), and makes use of state->speed and
state->duplex in contravention of the phylink documentation. This makes
reliant on the legacy behaviours, so mark it as a legacy driver.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>