superio: do not assume that current LDN cannot change after config exit

That assumption should be true when superio(4) uses the hardware
exlusively.  But it turns out to not hold on some real systems.
So, err on the side of correctness rather than performance.
Clear current_ldn in sio_conf_exit.

Reported by:	bz
Tested by:	bz
MFC after:	1 week

sys/dev/superio/superio.c

@@ -190,6 +190,7 @@ static void
 sio_conf_exit(struct siosc *sc)
 {
 	sc->methods->exit(sc->io_res, sc->io_port);
+	sc->current_ldn = 0xff;
 	mtx_unlock(&sc->conf_lock);
 }