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: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>