RDMA/cxgb4: Declare stag as __be32

The scqe.stag is actually __b32, fix it.

  drivers/infiniband/hw/cxgb4/cq.c:754:52: warning: cast to restricted __be32

Cc: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>