net/mlx5e: Separate ethtool_steering from fs.h and make private

Create a new fs_ethtool.h header file, where ethtool steering init and
cleanup functions are declared in it.
Make mlx5e_ethtool_steering struct private and declare at en_fs_ethtool.c.

Signed-off-by: Lama Kayal <lkayal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>