mlx5: support RSS hash update and get

mlx5: support RSS hash update and get

First implementation of rss_hash_update and rss_hash_conf_get, those
functions still lack in functionality but are usable to change the RSS
hash key.  For now, the PMD does not handle an indirection table for
each kind of flow (IPv4, IPv6, etc.), the same RSS hash key is used
for all protocols.  This situation explains why the rss_hash_conf_get
returns the RSS hash key for all DPDK supported protocols and why the
hash key is set for all of them too.

Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>