ACPI: sleep: Fix GPE suspend cleanup

Commit 9b039330

 removed
acpi_gpe_sleep_prepare(), the only function used at S5 transition
Add call to generic acpi_enable_wake_device().

Reference: https://bugzilla.novell.com/show_bug.cgi?id=299882

Signed-off-by: Alexey Starikovskiy <astarikovskiy@suse.de>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Len Brown <len.brown@intel.com>