ptp: xgbe: convert to .adjfine and adjust_by_scaled_ppm

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

Convert this driver to .adjfine and use adjust_by_scaled_ppm to calculate
the new addend value.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>