net/mlx4_core: Remove MCG in case it is attached to promiscuous QPs only

In B0 steering mode if promiscuous QP asks to be detached from MCG entry,
and it is the only one in this entry then the entry will never be deleted.
This is a wrong behavior since we don't want to keep those entries after
the promiscuous QP becomes non-promiscuous. Therefore remove steering
entry containing only promiscuous QP.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>