buffet: Set reg_status to 'invalid_credentials' on deregistration

In order for JS to power wash the device on deregistration, its device
registration status should be set to 'invalid_credentials.

BUG=brillo:1228
TEST=`FEATURES=test emerge-link buffet`
     `test_that -b link <ip> e:buffet_.*`

Change-Id: I2b55e5948b57cb8b7f3746378b7479046fd8f00a
Reviewed-on: https://chromium-review.googlesource.com/282385
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Reviewed-by: Aaron Kemp <kemp@google.com>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/buffet/device_registration_info.cc b/buffet/device_registration_info.cc
index 779a57d..4b74d7a 100644
--- a/buffet/device_registration_info.cc
+++ b/buffet/device_registration_info.cc
@@ -1073,6 +1073,7 @@
     pull_channel_.reset();
   }
   notification_channel_starting_ = false;
+  SetRegistrationStatus(RegistrationStatus::kInvalidCredentials);
 }
 
 }  // namespace buffet