sfc: Improve reliability of RX queue flushing

Reconfiguring the port requires us to flush all DMA queues.  In
repeated testing we have found that RX flushes would sometimes fail
because the RX DMA engine was not properly isolated from the MACs.
Signed-off-by: NBen Hutchings <bhutchings@solarflare.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/sfc/efx.c

@@ -1179,6 +1179,8 @@ static void efx_stop_all(struct efx_nic *efx)
 
 	/* Isolate the MAC from the TX and RX engines, so that queue
 	 * flushes will complete in a timely fashion. */
+	falcon_deconfigure_mac_wrapper(efx);
+	msleep(10); /* Let the Rx FIFO drain */
 	falcon_drain_tx_fifo(efx);
 
 	/* Stop the kernel transmit interface late, so the watchdog