carl9170: Use scnprintf() for avoiding potential buffer overflow

Since snprintf() returns the would-be-output size instead of the
actual output size, the succeeding calls may go beyond the given
buffer limit.  Fix it by replacing with scnprintf().

Cc: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ath/carl9170/debug.c

@@ -45,7 +45,7 @@
 #include "cmd.h"
 
 #define ADD(buf, off, max, fmt, args...)				\
-	off += snprintf(&buf[off], max - off, fmt, ##args);
+	off += scnprintf(&buf[off], max - off, fmt, ##args);
 
 
 struct carl9170_debugfs_fops {