ServoPowerStateController: Fix typo

commit b66e5c8dc0801 changed how servo is driven, replacing many calls
to self.controller.power_off() with self.host.power_off_via_servo().

One of those calls was changed to self.controller.power_off_via_servo()
which doesn't seem right.

BUG=b:171396005
TEST=none

Change-Id: Id567406494dcb43693c3b6e6855dbb563fcfa98d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/2490560
Reviewed-by: Rob Barnes <robbarnes@google.com>
Reviewed-by: Gregory Nisbet <gregorynisbet@google.com>
Reviewed-by: Derek Beckett <dbeckett@chromium.org>
Reviewed-by: Garry Wang <xianuowang@chromium.org>
Reviewed-by: Otabek Kasimov <otabek@google.com>
Tested-by: Rob Barnes <robbarnes@google.com>
Commit-Queue: Rob Barnes <robbarnes@google.com>
diff --git a/server/site_tests/platform_ServoPowerStateController/platform_ServoPowerStateController.py b/server/site_tests/platform_ServoPowerStateController/platform_ServoPowerStateController.py
index 635cd36..33ec8c4 100644
--- a/server/site_tests/platform_ServoPowerStateController/platform_ServoPowerStateController.py
+++ b/server/site_tests/platform_ServoPowerStateController/platform_ServoPowerStateController.py
@@ -161,7 +161,7 @@
         """Run test when USB stick is plugged in to servo."""
 
         logging.info('Power off DUT')
-        self.controller.power_off_via_servo()
+        self.host.power_off_via_servo()
         self.assert_dut_off('power_state:off did not turn DUT off.')
         # Servo V4 needs to be in snk role to allow booting from USB in
         # recovery mode (b/161464597).