net: dsa: change scope of bridging code

Now that the bridge join and leave functions only deal with a DSA port,
change their scope from the DSA slave net_device to the DSA generic
dsa_port.

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