Use the /sys/power/autosleep file to control the system power state.

The current wakelocks code doesn't prohibit explicit suspends from
the sys/power/state interface, but instead they block the autosleep code
from putting the system to sleep, if its enabled.

See:
https://lwn.net/Articles/479841/

and:
Documentation/ABI/testing/sysfs-power

Signed-off-by: Max Liao <liaops@embedinfo.com>