PM / devfreq: Do not show statistics if it's not ready.

Before this patch for a device without statistics support,

$ cat trans_stat
From : To
: time(ms)
Total transitions : 0
$

After this patch applied for such a device,

$ cat trans_stat
Not Supported.
$

Signed-off-by: MyungJoo Ham <[e-mail]>

drivers/devfreq/devfreq.c

···
 	if (!devfreq->stop_polling &&
 			devfreq_update_status(devfreq, devfreq->previous_freq))
 		return 0;
+	if (max_state == 0)
+		return sprintf(buf, "Not Supported.\n");
 
 	len = sprintf(buf, "     From  :   To\n");
 	len += sprintf(buf + len, "           :");