net: dsa: setup and teardown ports

The dsa_dsa_port_apply and dsa_cpu_port_apply functions do exactly the
same. The dsa_user_port_apply function does not try to register a fixed
link but try to create a slave.

This commit factorizes and scopes all that in two convenient
dsa_port_setup and dsa_port_teardown functions.

It won't hurt to register a devlink_port for unused port as well.

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