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

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

Fixes: 427c1e7b

 ("{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>