mlxsw: spectrum: Use existing flood setup when adding VLANs

When a VLAN is added on a bridge port we should use the existing unicast
flood configuration of the port instead of assuming it's enabled.

Fixes: 0293038e ("mlxsw: spectrum: Add support for flood control")
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>