net/mlx5e: Add CONFIG_MLX5_EN_RXNFC for ethtool rx nfc

Add new mlx5 Kconfig flag to allow selecting ethtool rx nfc support,
and compile out en_fs_ehtool.c if not selected.

Add en/fs.h header file to host all steering declarations and
definitions.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Moshe Shemesh <moshe@mellanox.com>