[ALSA] usbaudio - Fix kobject_add() error at reconnection

Fixed the error from kobject_add() at reconnection the usb audio device.
This happens when an app keeps opening a device while the device is
replugged, due to the confliction of the internal bookkept index and
the really empty slot.

Signed-off-by: Takashi Iwai <[e-mail]>
Signed-off-by: Jaroslav Kysela <[e-mail]>

sound/usb/usbaudio.c

···
 
 static int snd_usb_audio_free(struct snd_usb_audio *chip)
 {
+	usb_chip[chip->index] = NULL;
 	kfree(chip);
 	return 0;
 }
···
 		list_for_each(p, &chip->mixer_list) {
 			snd_usb_mixer_disconnect(p);
 		}
-		usb_chip[chip->index] = NULL;
 		mutex_unlock(&register_mutex);
 		snd_card_free_when_closed(card);
 	} else {