bridge: call NETDEV_JOIN notifiers when add a slave

In the previous patch I added NETDEV_JOIN, now
we can notify netconsole when adding a device to a bridge too.

Signed-off-by: WANG Cong <amwang@redhat.com>
Cc: Neil Horman <nhorman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>