client/cros: Fix incorrect wakealarm write

wakealarm expects seconds as input instead of literal 'clear' according
to documentation:
https://github.com/torvalds/linux/blob/master/Documentation/ABI/testing/sysfs-class-rtc

BUG=b:147278281
TEST=manual, invoked set_wake_alarm() several times and no more IOError

Change-Id: I400a0b47e25a5c5753507d7310c39a4ec2cab92a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/1990875
Reviewed-by: Pi-Hsun Shih <pihsun@chromium.org>
Reviewed-by: Nicolas Boichat <drinkcat@chromium.org>
Commit-Queue: Fei Shao <fshao@chromium.org>
Tested-by: Fei Shao <fshao@chromium.org>
diff --git a/client/cros/rtc.py b/client/cros/rtc.py
index 4b0c0f2..505fd71 100644
--- a/client/cros/rtc.py
+++ b/client/cros/rtc.py
@@ -42,5 +42,5 @@
     except IOError as (errnum, strerror):
         if errnum != errno.EBUSY:
             raise
-        write_wake_alarm('clear', rtc_device)
+        write_wake_alarm('0', rtc_device)
         write_wake_alarm(alarm_time, rtc_device)