Input: wacom - fix touch support for Bamboo Fun CTH-461

Commit f393ee2b814e3291c12565000210b3cf10aa5c1d forgot to add the
touch_max property for Wacom Bamboo Fun CTH-461/S, ID 056a:00d2.

This broke the touch functionality for that device. This patch,
(done with help of Ping Cheng), adds the correct value and makes
touch work again.

Signed-off-by: Diego Calleja <diegocg@gmail.com>
Reviewed-by: Ping Cheng <pinglinux@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>