IB/mlx5: Honor cnt_set_id_valid flag instead of set_id

It is incorrect to depend on set_id value to know if counters were
allocated or not. set_id_valid field is set to true when counters
were allocated. Therefore, use set_id_valid while deciding to
free counters.

Cc: <stable@vger.kernel.org> # 4.15
Fixes: aac4492e ("IB/mlx5: Update counter implementation for dual port RoCE")
Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>