IB/hfi1: Fix a build warning for TID RDMA READ

The following build warning was produced for the TID RDMA READ
patch ("IB/hfi1: Enable TID RDMA READ protocol"):

drivers/infiniband/hw/hfi1/qp.c: In function 'hfi1_setup_wqe':
drivers/infiniband/hw/hfi1/qp.c:328:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   hfi1_setup_tid_rdma_wqe(qp, wqe);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/infiniband/hw/hfi1/qp.c:329:2: note: here
  case IB_QPT_UC:
  ^~~~

This patch will fix the issue by adding the "fall through" comment.

Fixes: f1ab4efa ("IB/hfi1: Enable TID RDMA READ protocol")
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Kaike Wan <kaike.wan@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>