Input: wacom_w8001 - w8001_MAX_LENGTH should be 13

Somehow the patch that added two-finger touch support forgot to update
W8001_MAX_LENGTH from 11 to 13.

Signed-off-by: Ping Cheng <pingc@wacom.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>