net: dsa: mv88e6xxx: update 88e6185 PCS driver to use neg_mode

Update the Marvell 88e6185 PCS driver to use neg_mode rather than the
mode argument to match the other updated PCS drivers.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://lore.kernel.org/r/E1rhosE-003yuc-FM@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>