dp83640: Include hash in timestamp/packet matching

Only using the message type and sequence id for matching timestamps
with packets is error prone, as multiple clients may very well be
sending packets with the same messagetype and timestamp at the same
time. Fix by extending the check to include the hash of bytes 20-29
(source id in PTPv2) that is provided with the timestamps.

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