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>
diff --git a/sound/pci/hda/hda_beep.c b/sound/pci/hda/hda_beep.c
index 960fd79..4de5bac 100644
--- a/sound/pci/hda/hda_beep.c
+++ b/sound/pci/hda/hda_beep.c
@@ -138,6 +138,7 @@
 
 		input_unregister_device(beep->dev);
 		kfree(beep);
+		codec->beep = NULL;
 	}
 }
 EXPORT_SYMBOL_HDA(snd_hda_detach_beep_device);