wdog: wd_gettime() return 0 when wdog has already expired

Signed-off-by: ligd <liguiding1@xiaomi.com>

sched/wdog/wd_gettime.c

@@ -75,7 +75,7 @@ sclock_t wd_gettime(FAR struct wdog_s *wdog)
             {
               delay -= wd_elapse();
               leave_critical_section(flags);
-              return delay;
+              return delay < 0 ? 0 : delay;
             }
         }
     }