IB/rxe: make rxe_unregister_device void

Since the function rxe_unregister_device always returns 0, it is changed
to void.

Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>