ALSA: bebob: Remove meaningless mutex_unlock()

Currently mutex_unlock() is called in module's cleanup function. But after
cleaned up, this mutex is automatically released. So this function call
is meaningless.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/firewire/bebob/bebob.c

@@ -465,7 +465,6 @@ static void __exit
 snd_bebob_exit(void)
 {
 	driver_unregister(&bebob_driver.driver);
-	mutex_destroy(&devices_mutex);
 }
 
 module_init(snd_bebob_init);