[ALSA] Unregister device files at disconnection

Orignally proposed by Sam Revitch <sam.revitch@gmail.com>.
Unregister device files at disconnection to avoid the futher accesses.
Also, the dev_unregister callback is removed and replaced with the
combination of disconnect + free.
A new function snd_card_free_when_closed() is introduced, which is
used in USB disconnect callback.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>

sound/usb/usbaudio.c

@@ -3499,7 +3499,7 @@ static void snd_usb_audio_disconnect(struct usb_device *dev, void *ptr)
 		}
 		usb_chip[chip->index] = NULL;
 		mutex_unlock(&register_mutex);
-		snd_card_free(card);
+		snd_card_free_when_closed(card);
 	} else {
 		mutex_unlock(&register_mutex);
 	}