avf: tx node fixes
avf: tx node fixes

- missing RSV bit set in descriptor
- wrong buffer offset

Change-Id: I8b138266652a30a50e4541c6344e4fe3dec4d1ca
Signed-off-by: Damjan Marion <9141bba8b2efed526e55cf796d48631af330ad98@cisco.com>

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