util/nvtutil: fix out of bounds error

the error would have never been triggered, because it never
went over 11, but if this code were to be copied elsewhere,
it would be problematic

util/nvmutil/nvmutil.c

@@ -220,7 +220,7 @@ hextonum(char chs)
 	static int macfd;
 	static uint8_t *rmac = NULL;
 	static int random;
-	if (random > 15) {
+	if (random > 11) {
 		close(macfd);
 		free(rmac);
 		rmac = NULL;