dp83640: Only wait for timestamps for packets with timestamping enabled.

In the packet timestamping function, check that the ptp version and
protocol of the packet matches what we have configured the hardware to
actually generate timestamps for, before looking/waiting for a timestamp.

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