bisect-kit: fix bug: servo_port is not a string

BUG=b:184105298
TEST=manually

Change-Id: Ie7f27c0eba5cba91c507ae0df39630f10b899d0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/bisect-kit/+/2833502
Tested-by: Zheng-Jie Chang <zjchang@chromium.org>
Auto-Submit: Zheng-Jie Chang <zjchang@chromium.org>
Reviewed-by: Kuang-che Wu <kcwu@chromium.org>
Commit-Queue: Kuang-che Wu <kcwu@chromium.org>
diff --git a/bisect_kit/cros_lab_util.py b/bisect_kit/cros_lab_util.py
index 1ea29a2..26b1ceb 100644
--- a/bisect_kit/cros_lab_util.py
+++ b/bisect_kit/cros_lab_util.py
@@ -788,6 +788,7 @@
     logger.debug('this DUT has no servo?')
     return False
 
+  servo_port = str(servo_port)
   try:
     util.ssh_cmd(servo_host, 'start', 'servod', 'PORT=' + servo_port)
   except subprocess.CalledProcessError: