ALSA: hda - Clear codec->beep at release

Clear codec->beep field in snd_hda_detach_beep_device() to be sure.
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/hda/hda_beep.c

@@ -138,6 +138,7 @@ void snd_hda_detach_beep_device(struct hda_codec *codec)
 
 		input_unregister_device(beep->dev);
 		kfree(beep);
+		codec->beep = NULL;
 	}
 }
 EXPORT_SYMBOL_HDA(snd_hda_detach_beep_device);