net: remove phylink_pcs .neg_mode boolean

As all PCS are using the neg_mode parameter rather than the legacy
an_mode, remove the ability to use the legacy an_mode. We remove the
tests in the phylink code, unconditionally passing the PCS neg_mode
parameter to PCS methods, and remove setting the flag from drivers.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1tidPn-0040hd-2R@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>