sfc: use netdev_rss_key_fill() helper

Use netdev_rss_key_fill() helper, as it provides better support for some
bonding setups.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>