ptp: cpts: convert .adjfreq to .adjfine

The cpts implementation of .adjfreq is implemented in terms of a
straight forward "base * ppb / 1 billion" calculation.

Convert this to the newer .adjfine, using the recently added
adjust_by_scaled_ppm helper function.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>