i40e: rename index to port to avoid confusion

The .index field of i40e_udp_port_config represents the udp port number.
Rename this variable to port so that it is more obvious.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>