sound: allow the unit search until 256 in sound_core.c

The upper limit of the available minors isn't necessarily 128 + unit,
but it's rather up to 256.  Fixing this allows more than 8 devices.
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/sound_core.c

@@ -361,7 +361,7 @@ int register_sound_special_device(const struct file_operations *fops, int unit,
 				  struct device *dev)
 {
 	const int chain = unit % SOUND_STEP;
-	int max_unit = 128 + chain;
+	int max_unit = 256;
 	const char *name;
 	char _name[16];