last_rx_timestamp should be updated only when rx time stamp is
read. Also it's only used with NICs that have per-interface time
stamping resources so it can be moved to adapter structure and
set in igb_ptp_rx_rgtstamp().
Signed-off-by: Jakub Kicinski <firstname.lastname@example.org>
Acked-by: Matthew Vick <email@example.com>
Tested-by: Aaron Brown <firstname.lastname@example.org>
Signed-off-by: Jeff Kirsher <email@example.com>