i40e: display priority_xon and priority_xoff stats

Display some more stats that were already being counted, to help users
understand when priority xon/xoff packets are being sent/received

Signed-off-by: Alice Michael <alice.michael@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>