net/mlx5e: Take SW parser code to a separate function

Refactor mlx5e_ipsec_set_swp() code, split the part which sets the eseg
software parser (SWP) offsets and flags, so it can be used in a
downstream patch by other mlx5e functionality which needs to set eseg
SWP.
The new function mlx5e_set_eseg_swp() is useful for setting swp for both
outer and inner headers. It also handles the special ipsec case of xfrm
mode transfer.

Signed-off-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>