net/mlx4_core: Make sure that negative array index isn't used

To make sure that the array index isn't used in the code with
negative value, we stop using the for loop integer iterator
outside of it.
>From now on use members count to swap the last QP with removed one.
Fix also the second occurrence of this flow in mlx4_qp_detach_common().
In mlx4_qp_detach_common() use members_count instead of
loop iterator outside of the for loop.

Signed-off-by: Dotan Barak <dotanb@dev.mellanox.co.il>
Reviewed-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
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>