motion: Fix comparison between signed and unsigned integer

If wait_us < 0, comparison against motion_min_interval actually fails,
and this negative wait_us causes task_wait_event() never returns if we
are not using any motion task event except the timer. The motion task
will then stop running and sensor data stay unchanged.

BRANCH=none
BUG=chrome-os-partner:54092
TEST=hardcode wait_us to a negative value before motion_min_interval check,
and see motion task is still running by EC console cmd timerinfo

Change-Id: Ic1e7ffeeb9d2ec1f5c5beb4387294014298123af
Signed-off-by: Koro Chen <koro.chen@mediatek.com>
Reviewed-on: https://chromium-review.googlesource.com/358332
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>

common/motion_sense.c

@@ -827,6 +827,9 @@ void motion_sense_task(void)
 			wait_us = motion_interval -
 				(ts_end_task.val - ts_begin_task.val);
 
+			/* and it cannnot be negative */
+			wait_us = MAX(wait_us, 0);
+
 			/*
 			 * Guarantee some minimum delay to allow other lower
 			 * priority tasks to run.