i40e: add function header for i40e_get_rxfh

Someone left this poor little function naked with no header.  This
dresses it up in a proper function header it deserves.

Signed-off-by: Alan Brady <alan.brady@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>