mlx4_core: use correct flag for unicast_promisc

Use MLX4_DEV_CAP_FLAG_VEP_UC_STEER for unicast_promisc_add/remove
Unicast entries were managed in wrong data structures.

Signed-off-by: Eugenia Emantayev <eugenia@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>