RDMA/mlx5: Fix memory leak in case we fail to add an IB device

Make sure the IB device is freed on failure.

Fixes: b5ca15ad

 ("IB/mlx5: Add proper representors support")
Signed-off-by: Mark Bloch <markb@mellanox.com>
Reviewed-by: Bodong Wang <bodong@mellanox.com>
Reviewed-by: Håkon Bugge <haakon.bugge@oracle.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>