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

Update xpcs to use neg_mode to configure whether inband negotiation
should be used. We need to update sja1105 as well as that directly
calls into the XPCS driver's config function.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1qA8Dt-00EaFS-W9@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>