net: dsa: mv88e6xxx: handle unknown duplex modes gracefully in mv88e6xxx_port_set_duplex

When testing another issue I faced the problem that
mv88e6xxx_port_setup_mac() failed due to DUPLEX_UNKNOWN being passed
as argument to mv88e6xxx_port_set_duplex(). We should handle this case
gracefully and return -EOPNOTSUPP, like e.g. mv88e6xxx_port_set_speed()
is doing it.

Fixes: 7f1ae07b

 ("net: dsa: mv88e6xxx: add port duplex setter")
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>