net/mlx5e: Move mlx5e_priv_flags into en_ethtool.c

Move the definition of mlx5e_priv_flags into en_ethtool.c because it's
only used there.

Fixes: 4e59e288 ("net/mlx5e: Introduce net device priv flags infrastructure")
Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>