sfc: check hash is valid before using it

On EF100, the RX hash field in the packet prefix may not be valid (e.g.
 if the header parse failed), and this is indicated by a one-bit flag
 elsewhere in the packet prefix.  Only call skb_set_hash() if the
 RSS_HASH_VALID bit is set.

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