net/mlx5: E-Switch, always set mc_promisc for allmulti vports

Set the mc_promisc flag also in the case of adding new mc address to
existing allmulti vport.

Fixes: a35f71f2 ('net/mlx5: E-Switch, Implement promiscuous rx modes vf request handling')
Signed-off-by: Mohamad Haj Yahia <mohamad@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>