IB/hfi1: Prevent null pointer dereference

If a context has not been assigned or assignment failed, pq may be NULL.
Move the unregister within the protection of the null check.

Reviewed-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>