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: Bert Kenward <bkenward@solarflare.com>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>