ixgbe: update the rss key on h/w, when ethtool ask for it

Currently ixgbe_set_rxfh() updates the rss_key copy in the driver
memory, but does not push the new value into the h/w. This commit
add a new helper for the latter operation and call it in
ixgbe_set_rxfh(), so that the h/w rss key value can be really
updated via ethtool.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>