PM / devfreq: fix private_data

The "private_date" field in struct devfreq_dev_status almost certainly
wants to be "private_data"; since there are no in-tree users of this
functionality, now seems like an easy time to make the fix.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>