net/mlx4_en: Fix panic during reboot

Fix a kernel panic that occurs as a result of an asynchronous event
handled in roce_gid_mgmt:
mlx4_en_get_drvinfo is called and accesses freed resources.

This happens in a shutdown flow only, since pci device is destroyed
while netdevice is still alive.

Fixes: c27a02cd ("mlx4_en: Add driver for Mellanox ConnectX 10GbE NIC")
Signed-off-by: NEugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: NTariq Toukan <tariqt@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/en_netdev.c

@@ -2201,6 +2201,7 @@ void mlx4_en_destroy_netdev(struct net_device *dev)
 
 	if (!shutdown)
 		free_netdev(dev);
+	dev->ethtool_ops = NULL;
 }
 
 static int mlx4_en_change_mtu(struct net_device *dev, int new_mtu)