sfc: implement per-queue TSO (hw_gso) stats

Use our existing TSO stats, which count enqueued TSO TXes.
Users may expect them to count completions, as tx-packets and
 tx-bytes do; however, these are the counters we have, and the
 qstats documentation doesn't actually specify.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>