ptp: update adjfreq callback description

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: stable@vger.kernel.org [v3.5+]
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>

include/linux/ptp_clock_kernel.h

@@ -54,7 +54,8 @@ struct ptp_clock_request {
  * clock operations
  *
  * @adjfreq:  Adjusts the frequency of the hardware clock.
- *            parameter delta: Desired period change in parts per billion.
+ *            parameter delta: Desired frequency offset from nominal frequency
+ *            in parts per billion
  *
  * @adjtime:  Shifts the time of the hardware clock.
  *            parameter delta: Desired change in nanoseconds.