ptp_ixp46x: convert .adjfreq to .adjfine

The ptp_ixp46x 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>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>