sfc: clean fallbacks between promisc/normal in efx_ef10_filter_sync_rx_mode

Separate functions for inserting individual and promisc filters; explicit
 fallback logic in efx_ef10_filter_sync_rx_mode(), in order not to overload
 the 'promisc' flag as also meaning "fall back to promisc".
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>