net/mlx5e: Prepare IPsec packet reformat code for tunnel mode

Refactor setup_pkt_reformat() function to accommodate future extension
to support tunnel mode.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>