Fix negative sleep time in display_HotPlugAtSuspend

BUG=chromium:408376
TEST=manually run the test on BIG and observe negative sleep time being caught.

Change-Id: Iee7baa137eec7e3bf31e80f90bcb1e2baa0ead6e
Reviewed-on: https://chromium-review.googlesource.com/222495
Reviewed-by: Wai-Hong Tam <waihong@chromium.org>
Commit-Queue: Hung-ying Tyan <tyanh@chromium.org>
Tested-by: Hung-ying Tyan <tyanh@chromium.org>
1 file changed