net: phylink: add PCS negotiation mode

PCS have to work out whether they should enable PCS negotiation by
looking at the "mode" and "interface" arguments, and the Autoneg bit
in the advertising mask.

This leads to some complex logic, so lets pull that out into phylink
and instead pass a "neg_mode" argument to the PCS configuration and
link up methods, instead of the "mode" argument.

In order to transition drivers, add a "neg_mode" flag to the phylink
PCS structure to PCS can indicate whether they want to be passed the
neg_mode or the old mode argument.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1qA8De-00EaFA-Ht@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>