Input: wacom - use only one interface with DTU-2231

The Wacom DTU-2231 tablet has two interfaces on its default configuration
and both have HID class, leading to the creation of two input devices
instead of one. Only the first one is used, so filter out the second.

Signed-off-by: Aristeu Rozanski <aris@redhat.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>