net: phylink: use legacy_pre_march2020

Use the legacy flag to indicate whether we should operate in legacy
mode. This allows us to stop using the presence of a PCS as an
indicator to the age of the phylink user, and make PCS presence
optional.

Legacy mode involves:
1) calling mac_config() whenever the link comes up
2) calling mac_config() whenever the inband advertisement changes,
   possibly followed by a call to mac_an_restart()
3) making use of mac_an_restart()
4) making use of mac_pcs_get_state()

All the above functionality was moved to a seperate "PCS" block of
operations in March 2020.

Update the documents to indicate that the differences that this flag
makes.

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