net: dsa: mv88e6xxx: remove unneeded cmode initialization

This partially reverts ed8fe202 ("net: dsa: mv88e6xxx: prevent
interrupt storm caused by mv88e6390x_port_set_cmode"). I missed
that chip->ports[].cmode is overwritten anyway by the cmode
caching in mv88e6xxx_setup().

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>