HID: wiimote: Add extension handler stubs

All supported extensions report data as 6 byte block. All DRMs with extension
data provide at least 6 extension bytes. Hence a generic handler for all
extension bytes is sufficient and can be called on all DRMs.

The handler distinguishes the input and passes it to the right handler. Motion+
passes data interleaved so we can have Motion+ and a regular extension enabled
simultaneously.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>