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: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>