net/mlx5e: Improve ethtool rxnfc callback structure

Don't choose who implements the rxnfc "get/set" callbacks according to
CONFIG_MLX5_EN_RXNFC, instead have the callbacks always available and
delegate to a function of a different driver module when needed
(en_fs_ethtool.c), have stubs in en/fs.h to fallback to when
en_fs_ethtool.c is compiled out, to avoid complications and ifdefs in
en_main.c.

Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>