net/mlx5e: Moves pflags to priv->params

pflags is a configuration parameter for the netdev, naturally it belongs
to priv->params.
Also introduce MLX5E_GET_PFLAG

Signed-off-by: Shaker Daibes <shakerd@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>