Do not handle MOD_SHUTDOWN equally to MOD_UNLOAD in sound kernel module.

MOD_SHUTDOWN is not an end of existence, and there is a life after it.
In particular, code previously called on MOD_SHUTDOWN grabbed lock and
deallocated unit numbering. That caused infinite wait loop if snd_uaudio
tried to destroy its PCM device after that point.

MFC after:	3 days

sys/dev/sound/pcm/sound.c

@@ -1415,7 +1415,6 @@ sound_modevent(module_t mod, int type, void *data)
 			pcmsg_unrhdr = new_unrhdr(1, INT_MAX, NULL);
 			break;
 		case MOD_UNLOAD:
-		case MOD_SHUTDOWN:
 			ret = sndstat_acquire(curthread);
 			if (ret != 0)
 				break;
@@ -1424,6 +1423,8 @@ sound_modevent(module_t mod, int type, void *data)
 				pcmsg_unrhdr = NULL;
 			}
 			break;
+		case MOD_SHUTDOWN:
+			break;
 		default:
 			ret = ENOTSUP;
 	}