sfc: Use filter index rather than ID for rps_flow_id table

efx->type->filter_insert() returns an ID rather than the index that
 efx->type->filter_async_insert() used to, which causes it to exceed
 efx->type->max_rx_ip_filters on some EF10 configurations, leading to out-
 of-bounds array writes.
So, in efx_filter_rfs_work(), convert this back into an index (which is
 what the remove call in the expiry path expects, anyway).

Fixes: 3af0f342 ("sfc: replace asynchronous filter operations")
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>