mlxsw: spectrum_router: Flood packets to router after RIF creation

If a packet ingress the router but can't be assigned an ingress RIF,
it's dropped.

Therefore, in the case of RIF configured on top of a bridge, it makes
sense to start flooding broadcast packets to the router only after the
RIF was created.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>