PM / Sleep: Fix build warning in sysfs.c for CONFIG_PM_SLEEP unset

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>