dp83640: Delay scheduled work.

Currently rx_timestamp_work reschedules itself as a regular workqueue item,
effectively causing it run constantly as long as there are packets left in
the queue. Fix by using delayed workqueue items, limiting it to run only
every two jiffies.

Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
Signed-off-by: David S. Miller <davem@davemloft.net>