net: dsa: mark DSA phylink as legacy_pre_march2020

The majority of DSA drivers do not make use of the PCS support, and
thus operate in legacy mode. In order to preserve this behaviour in
future, we need to set the legacy_pre_march2020 flag so phylink knows
this may require the legacy calls.

There are some DSA drivers that do make use of PCS support, and these
will continue operating as before - legacy_pre_march2020 will not
prevent split-PCS support enabling the newer phylink behaviour.

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