enic: use netdev_rss_key_fill() helper

Use of well known RSS key might increase attack surface.

Switch to a random one, using generic helper so that all
ports share a common key.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Christian Benvenuti <benve@cisco.com>
Cc: Govindarajulu Varadarajan <_govind@gmx.com>
Cc: Sujith Sankar <ssujith@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>