DPDK uses a structure to get or set a new hash key (see
eth_rte_rss_hash_conf). rss_hf field from this structure is used in
rss_hash_get_conf to retrieve the hash key and in rss_hash_update uses
it to verify the key exists before trying to update it.
Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>