ptp: dp83640: Use the high resolution frequency method.

The dp83640 has a frequency resolution of about 0.029 ppb.
This patch lets users of the device benefit from the
increased frequency resolution when tuning the clock.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>