sfc: account XDP TXes in netdev base stats

When we handle a TX completion for an XDP packet, it is not counted
 in the per-TXQ netdev stats.  Record it in new internal counters,
 and include those in the device-wide total in efx_get_base_stats().

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>