IB/hfi1: Fix potential panic with sdma drained mechanism

The guard is backwards, potentially causing the SDMA client
to panic if a wait structure was not specified.

psm and verbs are not exposed to the issue, but fix the
code just to be correct.

Fixes: a545f530 ("staging/rdma/hfi: fix CQ completion order issue")
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>