ACPI: Drop device flag wake_capable

The wake_capable ACPI device flag is not necessary, because it is
only used in scan.c for recording the information that _PRW is
present for the given device.  That information is only used by
acpi_add_single_object() to decide whether or not to call
acpi_bus_get_wakeup_device_flags(), so the flag may be dropped
if the _PRW check is moved to acpi_bus_get_wakeup_device_flags().
Moreover, acpi_bus_get_wakeup_device_flags() always returns 0,
so it really should be void.

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