staging/rdma/hfi1: Clean up register device

Now that rdmavt has solidified in its design we can clean up the driver
specific register device functions. This handles hfi1.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>