HID: asus: Add T100CHI bluetooth keyboard dock special keys mapping

The Asus Transformer T100CHI comes with a Bluetooth keyboard dock which
uses the same 0xff31 Asus vendor HUT page as other Asus keyboards.

This commit adds its device-id to hid-asus and fixes an issue in the
descriptor of the 0xff31 Usage, which together fixes the special keys
on this keyboard not working.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>