px4_qurt_tasks: fix timeout calculation

It was wrong to subtract seconds and nanoseconds from each other. This
will wrap, therefore use ts_to_abstime and do the subtraction
afterwards.

src/platforms/qurt/px4_layer/px4_qurt_tasks.cpp

@@ -43,6 +43,7 @@
 #include "px4_workqueue.h"
 #include "px4_time.h"
 #include "hrt_work.h"
+#include <drivers/drv_hrt.h>
 #include <unistd.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -371,10 +372,7 @@ int px4_sem_timedwait(px4_sem_t *sem, const struct timespec *ts)
 	work_s _hpwork = {};
 
 	// We get an absolute time but want to calculate a timeout in us.
-	struct timespec ts_now;
-	px4_clock_gettime(CLOCK_REALTIME, &ts_now);
-	uint32_t timeout_us = ((ts->tv_sec - ts_now.tv_sec) * 1000000)
-			    + ((ts->tv_nsec - ts_now.tv_nsec) / 1000);
+	hrt_abstime timeout_us = ts_to_abstime((struct timespec *)ts) - hrt_absolute_time();
 
 	// Create a timer to unblock.
 	hrt_work_queue(&_hpwork, (worker_t)&timer_cb, (void *)sem, timeout_us);