net: fix L2TPv2 common header

net: fix L2TPv2 common header

The fields of L2TPv2 common header were reversed in big endian and
little endian.

This patch fixes this error to ensure L2TPv2 can be parsed correctly.

For L2TP reference:
https://datatracker.ietf.org/doc/html/rfc2661#section-3.1

Fixes: 3a929df1f286 ("ethdev: support L2TPv2 and PPP procotol")
Cc: stable@dpdk.org
Signed-off-by: Jie Wang <jie1x.wang@intel.com>
Acked-by: Ori Kam <orika@nvidia.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>