ethdev: allow to set RSS hash computation flags and/or key

ethdev: allow to set RSS hash computation flags and/or key

1) Add a new function "rss_hash_update" in the PMD API to dynamically
   update the RSS flags and/or the RSS key used by a NIC to compute the RSS
   hash of input packets.
   The new function uses the existing data structure "rte_eth_rss_conf" for
   the argument that contains the new hash flags and/or the new hash key to
   use.

2) Add the ixgbe-specific function "ixgbe_dev_rss_hash_update" and the
   igb-specific function "eth_igb_rss_hash_update" to update the RSS
   hash configuration of ixgbe and igb controllers respectively.
   Before changing anything, these 2 functions check that the update RSS
   operation does not attempt to disable RSS, if RSS was enabled at port
   initialization time, or does not attempt to enable RSS, if RSS was
   disabled at port initialization time.

Note:
   Configuring the RSS hash flags and the RSS key used by a NIC consists in
   updating appropriate PCI registers of the NIC.
   These operations have been manually tested with the interactive commands
   "write reg" and "write regbit" of the testpmd application.

Signed-off-by: Ivan Boule <ivan.boule@6wind.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>