net: dsa: Make use of devlink port flavour unused

If a port is unused, still create a devlink port for it, but set the
flavour to unused. This allows us to attach devlink regions to the
port, etc.

Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Tested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>