net/mlx4_core: Change the default value of enable_qos

Change the default status of quality of service back to disabled,
as it hurts performance in some cases.

Fixes: 38438f7c

 ("net/mlx4: Set enhanced QoS support by default when ...")
Signed-off-by: Moshe Lazer <moshel@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>