Commit 64d47d50 ("net: dsa: mv88e6xxx: configure interface settings
in mac_config") removed forcing of speed and duplex from
mv88e6xxx_mac_config(), where the link is forced down, and left it only
in mv88e6xxx_mac_link_up(), by which time link is unforced.

It seems that (at least on 88E6190) when changing cmode to 2500base-x,
if the link is not forced down, but the speed or duplex are still
forced, the forcing of new settings for speed & duplex doesn't take in
mv88e6xxx_mac_link_up().

Fix this by unforcing speed & duplex in mv88e6xxx_mac_link_down().

Fixes: 64d47d50 ("net: dsa: mv88e6xxx: configure interface settings in mac_config")
Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>