net: dsa: mv88e6xxx: force cmode write on 6141/6341

mv88e6xxx_port_set_cmode() relies on cmode stored in struct
mv88e6xxx_port to skip cmode update when the requested value matches the
cached value. It turns out that mv88e6xxx_port_hidden_write() might
change the port cmode setting as a side effect, so we can't rely on the
cached value to determine that cmode update in not necessary.

Force cmode update in mv88e6341_port_set_cmode(), to make
serdes configuration work again. Other mv88e6xxx_port_set_cmode()
callers keep the current behaviour.

This fixes serdes configuration of the 6141 switch on SolidRun Clearfog
GT-8K.

Fixes: 7a3007d2 ("net: dsa: mv88e6xxx: fully support SERDES on Topaz family")
Reported-by: Denis Odintsov <d.odintsov@traviangames.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>