net: dsa: Add a ports structure and use it in the switch structure

net: dsa: Add a ports structure and use it in the switch structure

There are going to be more per-port members added to the switch
structure. So add a port structure and move the netdev into it.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>