net: dsa: mv88e6xxx: move link forcing to mac_prepare/mac_finish

Move the link forcing out of mac_config() and into the mac_prepare()
and mac_finish() methods. This results in no change to the order in
which these operations are performed, but does mean when we convert
mv88e6xxx to phylink_pcs support, we will continue to preserve this
ordering.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>