sfc: Move siena_nic_data::ipv6_rss_key to efx_nic::rx_hash_key

We will use this hash key for Toeplitz IPv4 hashing too.

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