net: dsa: use a single switch statement for port setup

It is currently difficult to read the different steps involved in the
setup and teardown of ports in the DSA code. Keep it simple with a
single switch statement for each port type: UNUSED, CPU, DSA, or USER.

Also no need to call devlink_port_unregister from within dsa_port_setup
as this step is inconditionally handled by dsa_port_teardown on error.

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