The Sixaxis does not want the report_id as part of the data packet in
Output reports, so we have to discard buf[0] when sending the actual
control message.

Add also some documentation about that and about why
hdev->hid_output_raw_report needs to be overridden.
Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>