net/mlx4_en: Disable blueflame using ethtool private flags

Enable the user to turn off the hardware feature called BlueFlame.
Since it is something specific to mlx4_en hardware, we control
the feature via ethtool private flags.

Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>