PM / devfreq: Correct spelling in a comment

The device attribute exposed in sysfs is called 'polling_interval'. Align
the comment.
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>

drivers/devfreq/governor.h

@@ -40,7 +40,7 @@
 /*
  * Definition of governor attribute flags except for common sysfs attributes
  * - DEVFREQ_GOV_ATTR_POLLING_INTERVAL
- *   : Indicate polling_interal sysfs attribute
+ *   : Indicate polling_interval sysfs attribute
  * - DEVFREQ_GOV_ATTR_TIMER
  *   : Indicate timer sysfs attribute
  */