IB/hfi1: Correct error calldown locking

The resource specific wait locking missed correcting the lock
for the notify_error_qp() calldown.

The code is fixed to correctly use the iowait lock field to protect
the head that is protected by that lock.

Fixes: Commit 4e045572 ("IB/hfi1: Add unique txwait_lock for txreq events")
Reviewed-by: Sebastian Sanchez <sebastian.sanchez@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>