sfc: Use a global count of active queues instead of pending drains

On EF10, the firmware will initiate a queue flush in certain
error cases.  We need to accept that flush events might appear
at any time after a queue has been initialised, not just when
we try to flush them.

We can handle Falcon-architecture in just the same way.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>