sfc: Fix two causes of flush failure

sfc: Fix two causes of flush failure

[ Upstream commits a606f4325dca6950996abbae452d33f2af095f39,
  d5e8cc6c946e0857826dcfbb3585068858445bfe,
  525d9e824018cd7cc8d8d44832ddcd363abfe6e1 ]

The TX DMA engine issues upstream read requests when there is room in
the TX FIFO for the completion. However, the fetches for the rest of
the packet might be delayed by any back pressure.  Since a flush must
wait for an EOP, the entire flush may be delayed by back pressure.

Mitigate this by disabling flow control before the flushes are
started.  Since PF and VF flushes run in parallel introduce
fc_disable, a reference count of the number of flushes outstanding.

The same principle could be applied to Falcon, but that
would bring with it its own testing.

We sometimes hit a "failed to flush" timeout on some TX queues, but the
flushes have completed and the flush completion events seem to go missing.
In this case, we can check the TX_DESC_PTR_TBL register and drain the
queues if the flushes had finished.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
[bwh: Backported to 3.2:
 - Call efx_nic_type::finish_flush() on both success and failure paths
 - Check the TX_DESC_PTR_TBL registers in the polling loop
 - Declare efx_mcdi_set_mac() extern]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>