net/mlx4_en: mlx4_en_[gs]et_priv_flags() can be static

Fixes sparse warning intrduced by commit 0fef9d03 ("net/mlx4_en: Disable
blueflame using ethtool private flags")

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