net: dsa: move bridge device in dsa_port

Move the bridge_dev pointer from dsa_slave_priv to dsa_port so that DSA
drivers can access this information and remove the need to cache it.

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