net: dsa: Do not register devlink for unused ports

Even if commit 1d27732f ("net: dsa: setup and teardown ports") indicated
that registering a devlink instance for unused ports is not a problem, and this
is true, this can be confusing nonetheless, so let's not do it.

Fixes: 1d27732f

 ("net: dsa: setup and teardown ports")
Reported-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>