firmware_ECUsbPorts: Check USB ports are disabled when powered off

When system is powered off, USB ports should be disabled. Let's check
this in USB port test.

BUG=chrome-os-partner:10292
TEST=Test passed

Change-Id: If8e0d255b9bfe1406872ca71945677bda38d1220
Reviewed-on: https://gerrit.chromium.org/gerrit/25726
Reviewed-by: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Vic Yang <victoryang@chromium.org>
Commit-Ready: Vic Yang <victoryang@chromium.org>
1 file changed