net/mlx4_core: Fail device init if num_vfs is negative

Should not allow negative num_vfs

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.com>
Signed-off-by: Vladimir Sokolovsky <vlad@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>