i40e: update comment explaining where FDIR buffers are freed

The original comment implies that the only location where the raw_packet
buffer will be freed is in i40e_clean_tx_ring() which is incorrect. In
fact this isn't even the normal case. Update the comment explaining
where the memory is freed.

Change-ID: Ie0defc35ed1c3af183f81fdc60b6d783707a5595
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>