timers/oneshot: Remove all private callback.

This commit removed all private callback and handle it on the upperhalf
of oneshot.

Signed-off-by: ouyangxiangzhen <ouyangxiangzhen@xiaomi.com>

sched/sched/sched_cpuload_oneshot.c

@@ -328,6 +328,8 @@ void nxsched_oneshot_extclk(FAR struct oneshot_lowerhalf_s *lower)
   /* Then start the oneshot */
 
   g_sched_oneshot.oneshot = lower;
+  lower->callback = nxsched_oneshot_callback;
+  lower->arg      = NULL;
   nxsched_oneshot_start();
 }
 #endif