libweave: Simplify NotifyPropertiesUpdated a little

Replaced code with smaller equivalent.

BUG=none
TEST='FEATURES=test emerge-gizmo buffet'

Change-Id: I735859b371b7e7abc45a8d5e915df7d16480d583
Reviewed-on: https://chromium-review.googlesource.com/289302
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>
Trybot-Ready: David James <davidjames@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>