IB/rdmavt/hfi1/qib: Use the MGID and MLID for multicast addressing

The Infiniband spec defines "A multicast address is defined by a
MGID and a MLID" (section 10.5).

The current code only uses the MGID for identifying multicast groups.
Update the driver to be compliant with this definition.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Dasaratharaman Chandramouli <dasaratharaman.chandramouli@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>