faft: Ensure DUT offline after the reboot of switching to dev/normal mode

Switching dev/normal mode needs reboot. Should wait for DUT surely offline
such that the firmware delay becomes more accurate.

BUG=chrome-os-partner:11694
TEST=Switch to run TryFwB/control.normal and TryFwB/control.dev passed.

Change-Id: I1be43a5a3f9a2d238da13ea53293296b8cc56fe3
Reviewed-on: https://gerrit.chromium.org/gerrit/28902
Reviewed-by: Vic Yang <victoryang@chromium.org>
Commit-Ready: Tom Wai-Hong Tam <waihong@chromium.org>
Reviewed-by: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Tom Wai-Hong Tam <waihong@chromium.org>
diff --git a/server/cros/faftsequence.py b/server/cros/faftsequence.py
index 7a6042e..7251aa7 100644
--- a/server/cros/faftsequence.py
+++ b/server/cros/faftsequence.py
@@ -791,6 +791,7 @@
         self.servo.set('usb_mux_sel1', 'servo_sees_usbkey')
         # Rebooting EC with rec mode on. Should power on AP.
         self.enable_rec_mode_and_reboot()
+        self.wait_for_client_offline()
         self.wait_fw_screen_and_switch_keyboard_dev_mode(dev=True)
 
 
@@ -798,6 +799,7 @@
         logging.info("Disabling keyboard controlled developer mode")
         self.servo.disable_recovery_mode()
         self.cold_reboot()
+        self.wait_for_client_offline()
         self.wait_fw_screen_and_switch_keyboard_dev_mode(dev=False)