ptp: lan743x: use diff_by_scaled_ppm in .adjfine implementation

Update the lan743x driver to use the recently added diff_by_scaled_ppm
helper function. This reduces the amount of code required in lan743x_ptp.c
driver file.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Cc: Bryan Whitehead <bryan.whitehead@microchip.com>
Cc: UNGLinuxDriver@microchip.com
Signed-off-by: David S. Miller <davem@davemloft.net>