ptp: update adjfreq callback description

commit 87f4d7c1d36f44b0822053b7e5dedc31fdd0ab99 upstream.

This patch updates the adjfreq callback description to include a note that the
delta in ppb is always relative to the base frequency, and not to the current
frequency of the hardware clock.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
CC: Richard Cochran <richard.cochran@gmail.com>
CC: John Stultz <john.stultz@linaro.org>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>