sfc: Enable RX scatter for flows steered by RFS

Received packets are only scattered if this is enabled in both the
matching filter and the receiving queue.  This was not being done for
filters inserted for RFS, so any packet requiring more than a single
descriptor was dropped.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>