Bug fix of queue empty slot calculation

Current implementation of queue_has_space doesn't handle the case where
queue tail has wrapped around the end. This CL fixes the bug.

BUG=None
TEST=Pass the test in the following CL.
BRANCH=link

Change-Id: I774f388081af50f0e930a6cbc3a723da1c8283b0
Signed-off-by: Vic Yang <victoryang@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/58031
Reviewed-by: Yung-Chieh Lo <yjlou@chromium.org>