net/iavf: rework Tx path

net/iavf: rework Tx path

Rework the Tx path and Tx descriptor usage in order to
allow for better use of offload flags and to facilitate enabling of
inline crypto offload feature.

Signed-off-by: Declan Doherty <declan.doherty@intel.com>
Signed-off-by: Abhijit Sinha <abhijit.sinha@intel.com>
Signed-off-by: Radu Nicolau <radu.nicolau@intel.com>
Acked-by: Jingjing Wu <jingjing.wu@intel.com>