ALSA: oss: underflow in snd_mixer_oss_proc_write()

We cap the upper bound of "idx" but not the negative side.  Let's make
it unsigned to fix this.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/oss/mixer_oss.c

@@ -1177,7 +1177,8 @@ static void snd_mixer_oss_proc_write(struct snd_info_entry *entry,
 	struct snd_mixer_oss *mixer = entry->private_data;
 	char line[128], str[32], idxstr[16];
 	const char *cptr;
-	int ch, idx;
+	unsigned int idx;
+	int ch;
 	struct snd_mixer_oss_assign_table *tbl;
 	struct slot *slot;