ptp: add missing documentation for parameters

The ptp_find_pin_unlocked function and the ptp_system_timestamp structure
didn't document their parameters and fields. Fix this.

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