net/mlx5e: IPoIB, Improve ethtool rxnfc callback structure in IPoIB

Followup commit
79ce39be ("net/mlx5e: Improve ethtool rxnfc callback structure")
and handle CONFIG_MLX5_EN_RXNFC enabled/disabled inside the fs layer so
the ethtool callbacks are always available. The fs layer will provide
stubs when CONFIG_MLX5_EN_RXNFC is compiled out.

Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>