ACPI / Sleep: Rework enabling wakeup devices

There is no reason why acpi_enable_wakeup_device() should be called
with interrupts disabled, because it doesn't access hardware.  Thus
it is possible to move it next to acpi_enable_wakeup_device_prep()
and make the ACPI suspend, hibernate and poweroff code more
straightforward.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Len Brown <len.brown@intel.com>