net/mlx4_en: really allow to change RSS key

When changing rss key, we do not want to overwrite user provided key
by the one provided by netdev_rss_key_fill(), which is the host random
key generated at boot time.

Fixes: 947cbb0a ("net/mlx4_en: Support for configurable RSS hash function")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Eyal Perry <eyalpe@mellanox.com>
CC: Amir Vadai <amirv@mellanox.com>
Acked-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>