Input: wacom - not all multi-interface devices support touch

Some multi-interface devices support expresskeys on a separate interface,
such as Bamboo; some multi-interface devices do not support touch at all,
such as Pen only Intuos5. Make sure we report the right device names.

Tested-by: Jason Gerecke <killertofu@gmail.com>
Signed-off-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>