schedule: dp: zero-initialise private data pointer

The LL tick task's .priv_data pointer has to be NULL for the check in
zephyr_ll_task_init() to pass.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

Author: lyakh

src/schedule/zephyr_dp_schedule.c

@@ -358,6 +358,7 @@ __cold int scheduler_dp_init(void)
 	if (!dp_sch)
 		return -ENOMEM;
 
+	dp_sch->ll_tick_src.priv_data = NULL;
 	list_init(&dp_sch->tasks);
 
 	scheduler_init(SOF_SCHEDULE_DP, &schedule_dp_ops, dp_sch);