PM / devfreq: make devfreq_class static

devfreq_class is used internally by devfreq and has no need to be globally available.

This also fixes the following sparse warning:
drivers/devfreq/devfreq.c:30:14: warning: symbol 'devfreq_class' was not declared. Should it be static?

---
 drivers/devfreq/devfreq.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 9f562ae..964e08d 100644
@@ -27,7 +27,7 @@
 #include <linux/hrtimer.h>
 #include "governor.h"
 
-struct class *devfreq_class;
+static struct class *devfreq_class;
 
 /*
  * devfreq_work periodically monitors every registered device.

1.7.9.5