sfc: support cascaded multicast filters

If the workaround to support cascaded multicast filters ("workaround_26807") is
enabled, the broadcast filter and individual multicast filters are not inserted
when in promiscuous or allmulti mode.

There is a race while inserting and removing filters when entering and leaving
promiscuous mode.  When changing promiscuous state with cascaded multicast
filters, the old multicast filters are removed before inserting the new filters
to avoid duplicating packets; this can lead to dropped packets until all
filters have been inserted.

The efx_nic:mc_promisc flag is added to record the presence of a multicast
promiscuous filter; this gives a simple way to tell if the promiscuous state is
changing.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>