PM / sysfs: avoid shadowing variables

The global variable "enabled" is shadowed in a number of
functions in this file, rename it to "_enabled" to avoid
that. For consistency, also rename "disabled" and move
them both into the #ifdef where they're needed.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>