net: pcs: lynxi: update PCS driver to use neg_mode

Update the Lynxi PCS driver to use neg_mode rather than the mode
argument. This ensures that the link_up() method will always program
the speed and duplex when negotiation is disabled.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1qA8Dz-00EaFY-5A@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>