IB/hfi1: Use accessor to determine ring size

The qp_stats print will soon be moving to rdmavt, so use the proper
accessor to get the ring size rather than a driver supplied constant.

Fixes: Commit ff8d836e

 ("IB/hfi1: Add receiving queue info to qp_stats")
Reviewed-by: Kaike Wan <kaike.wan@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>