ptp: remove the .adjfreq interface function

Now that all drivers have been converted to .adjfine, we can remove the
.adjfreq from the interface structure.

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