IB/rdmavt: Correct warning during QPN allocation

Correct calculation of the low order bits which should be unset
based on use of qos_shift parameter when assigning QPN.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Brian Welty <brian.welty@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>