ACPI: WMI: Enable event methods when registering notifiers

According to the ACPI-WMI spec, event blocks may provide a function call
for enabling/disabling them. This patch adds support for making these
calls when registering or removing notifications. Without this, my Dell
firmware provides no data in the event notification.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Carlos Corbacho <carlos@strangeworlds.co.uk>
Signed-off-by: Len Brown <len.brown@intel.com>