The power/async device sysfs attribute is only used if both
CONFIG_PM_ADVANCED_DEBUG and CONFIG_PM_SLEEP are set, but the code
implementing it doesn't depend on CONFIG_PM_SLEEP. As a result, a
build warning appears if CONFIG_PM_ADVANCED_DEBUG is set and
CONFIG_PM_SLEEP is not set.
Fix it by adding a #ifdef CONFIG_PM_SLEEP around the code in
question.
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>