IB/mlx5: Fix an error code in __mlx5_ib_modify_qp()

"err" is either zero or possibly uninitialized here.  It should be
-EINVAL.

Fixes: 427c1e7 ("{IB, net}/mlx5: Move the modify QP operation table to mlx5_ib")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/hw/mlx5/qp.c

@@ -3100,8 +3100,10 @@ static int __mlx5_ib_modify_qp(struct ib_qp *ibqp,
 		goto out;
 
 	if (mlx5_cur >= MLX5_QP_NUM_STATE || mlx5_new >= MLX5_QP_NUM_STATE ||
-	    !optab[mlx5_cur][mlx5_new])
+	    !optab[mlx5_cur][mlx5_new]) {
+		err = -EINVAL;
 		goto out;
+	}
 
 	op = optab[mlx5_cur][mlx5_new];
 	optpar = ib_mask_to_mlx5_opt(attr_mask);