dp83640: Fix receive timestamp race condition

When timestamping received packets, rx_timestamp_work may be scheduled
before the timestamps is received from the hardware resulting in the
packet beeing delivered without the timestamp.

This is fixed by changing the receive timestamp path:

On receiving a packet that need timestamping, the rxts list is
traversed.  If a match is found, packet+timestamp are delivered,
otherwise the packet is added to a rx_queue.

When a timestamp arrives rx_queue is traversed and if a matching
packet is found, it is delivered with the timestamp. Otherwise the
timestamp is added to the rxts list for matching with packets arriving
later.

In case the hardware drops a timestamp, a workqueue regularly checks
the queue for old packets and delivers them without a timestamp.

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