ACPI: Introduce new device wakeup flag 'prepared'

Introduce additional flag 'prepared' in struct acpi_device_wakeup_flags
and use it to prevent devices from being enable/disabled do wake up the
system multiple times in a row (this does not happen currently, but will
be possible after some of the following patches).

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>