mlx5_core: Fix use after free in mlx5_cmd_comp_handler()

We can't dereference "ent" after passing it to free_cmd().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>