net: dsa: mv88e6xxx: setup port's MAC

Now that we have setters to configure the port's MAC, use them to
refactor the port setup and adjust_link code.

Note that port's MAC speed, duplex or RGMII delay must not be changed
unless the port's link is forced down. So wrap all that in a
mv88e6xxx_port_setup_mac function.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>