net: dsa: mv88e6xxx: keep CMODE writable code private

This is a follow-up patch for commit 7a3007d2 ("net: dsa:
mv88e6xxx: fully support SERDES on Topaz family").

Since .port_set_cmode is only called from mv88e6xxx_port_setup_mac and
mv88e6xxx_phylink_mac_config, it is fine to keep this "make writable"
code private to the mv88e6341_port_set_cmode implementation, instead
of adding yet another operation to the switch info structure.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>