net: pcs: rzn1-miic: update PCS driver to use neg_mode

Update the RZN1-MIIC 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>
Link: https://lore.kernel.org/r/E1rhos9-003yuW-Az@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>