net/mlx4_core: double free of dev_vfs

If user loads mlx4_core with num_vfs greater than
supported then variable dev->dev_vfs is freed 2 times after unloading the
driver.

Acked-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Carol L Soto <clsoto@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>