netif_tx_napi_add() is a variant of netif_napi_add()

It should be used by drivers that use a napi structure
to exclusively poll TX.

We do not want to add this kind of napi in napi_hash[] in following
patches, adding generic busy polling to all NAPI drivers.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>