SUNRPC: Clean up unused variable in rpc_xprt_probe_trunked()

We don't use the rpc_xprt_switch anywhere in this function, so let's not
take an extra reference to in unnecessarily.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>