sfc: don't rearm interrupts if busy polling

Since commit 364b6055 ("net: busy-poll: return busypolling status
to drivers"), napi_complete_done() returns a boolean that can be used
by drivers to conditionally rearm interrupts.

Testing with a 7142 shows a small latency improvement of ~100 ns.
Signed-off-by: NBert Kenward <bkenward@solarflare.com>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/sfc/efx.c

@@ -328,7 +328,7 @@ static int efx_poll(struct napi_struct *napi, int budget)
 		 * since efx_nic_eventq_read_ack() will have no effect if
 		 * interrupts have already been disabled.
 		 */
-		napi_complete_done(napi, spent);
+		if (napi_complete_done(napi, spent))
 			efx_nic_eventq_read_ack(channel);
 	}