IB/hfi1: Check xchg returned value for queuing link down entry

Check xchg returned value for queuing link down entry
to guarantee proper atomic value reads.

Fixes: 626c077c

 ("IB/hfi1: Prevent link down request double queuing")
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Sebastian Sanchez <sebastian.sanchez@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>