Fix an error in clock_timespec_subtract

ChangeLog

@@ -11058,4 +11058,5 @@
 	* drivers/discrete/userled_lower.c:  Add a generic lower-half user LED
 	  driver that may be used by any board that supports the standard
 	  board user LED interfaces (2015-11-01).
-
+	* sched/clock: Fix error in clock_timespec_subtract().  Found by Lok
+	  (2015-11-03).

sched/clock/clock_timespec_subtract.c

@@ -83,7 +83,7 @@ void clock_timespec_subtract(FAR const struct timespec *ts1,
     }
   else
     {
-      sec = ts1->tv_sec + ts2->tv_sec;
+      sec = ts1->tv_sec - ts2->tv_sec;
       if (ts1->tv_nsec < ts2->tv_nsec)
         {
           nsec = (ts1->tv_nsec + NSEC_PER_SEC) - ts2->tv_nsec;