selftests: mlxsw: devlink_trap_tunnel_ipip: Remove code duplication

As part of adding same test for GRE tunnel with IPv6 underlay, an
optional improvement was found - call ipip_payload_get from
ecn_payload_get, so do not duplicate the code which creates the payload.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>