svcrdma: do not unregister device for listeners

On an rdma-capable machine, a start/stop/start and then on a stop of
a knfsd server would lead kref underflow warning because svc_rdma_free
would indiscriminately unregister the rdma device but a listening
transport never calls the rdma_rn_register() thus leading to kref
going down to 0 on the 1st stop of the server and on the 2nd stop
it leads to a problem.

Suggested-by: Chuck Lever <chuck.lever@oracle.com>
Fixes: c4de97f7 ("svcrdma: Handle device removal outside of the CM event handler")
Signed-off-by: Olga Kornievskaia <okorniev@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>