net/mlx5e: IPsec, Inline feature_check fast-path function

Feature check functions are in the TX fast-path of all SKBs, not only
IPsec traffic.
Move the IPsec feature check function into a header and turn it inline.
Use a stub and clean the config flag condition in Eth main driver file.

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Raed Salem <raeds@nvidia.com>
Reviewed-by: Huy Nguyen <huyn@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>