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: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/sound_core.c b/sound/sound_core.c
index c6e81fb..fb9255c 100644
--- a/sound/sound_core.c
+++ b/sound/sound_core.c
@@ -361,7 +361,7 @@
 				  struct device *dev)
 {
 	const int chain = unit % SOUND_STEP;
-	int max_unit = 128 + chain;
+	int max_unit = 256;
 	const char *name;
 	char _name[16];