sfc: use the new __netdev_tx_sent_queue BQL optimisation

As added in 3e59020a

 ("net: bql: add __netdev_tx_sent_queue()"), which
 see for performance rationale.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>