dp83640: Prune rx timestamp list before reading from it

The list of rx timestamps are currently only pruned of old entries when a
new entry is inserted. If no new entries are added, old timestamps may
survive beyond their lifetime, possible causing them to be attached to
packets with the same sequence number after a rollover.

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