ptp: tg3: convert .adjfreq to .adjfine

The tg3 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
diff_by_scaled_ppm helper function to calculate the difference and
direction of the adjustment.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Cc: Siva Reddy Kallam <siva.kallam@broadcom.com>
Cc: Prashant Sreedharan <prashant@broadcom.com>
Cc: Michael Chan <mchan@broadcom.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>