net: dsa: don't do devlink port setup early

Commit 3122433e

 ("net: dsa: Register devlink ports before calling DSA driver setup()")
moved devlink port setup to be done early before driver setup()
was called. That is no longer needed, so move the devlink port
initialization back to dsa_port_setup(), as the first thing done there.

Note there is no longer needed to reinit port as unused if
dsa_port_setup() fails, as it unregisters the devlink port instance on
the error path.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>