bridge: Introduce BR_PROMISC flag

Introduce a BR_PROMISC per-port flag that will help us track if the
current port is supposed to be in promiscuous mode or not.  For now,
always start in promiscuous mode.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>