Fix TCP checksum.
Fix TCP checksum.

Set TCP checksum to zero, before calculate the TCP checksum.

Change-Id: Id96743334481804b8ebb74afef2f1dbfed29e0cb
Signed-off-by: Andrej Kozemcak <806670a1cb1f0508258ef96648425f1247742d81@pantheon.tech>

vpp-dev/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,FDio/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp,chrisy/vpp,chrisy/vpp