libeth: add Tx buffer completion helpers

Software-side Tx buffers for storing DMA, frame size, skb pointers etc.
are pretty much generic and every driver defines them the same way. The
same can be said for software Tx completions -- same napi_consume_skb()s
and all that...
Add a couple simple wrappers for doing that to stop repeating the old
tale at least within the Intel code. Drivers are free to use 'priv'
member at the end of the structure.

Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>