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 <myungjoo.ham@samsung.com>