Input: wacom - fix "can not retrieve extra class descriptor" for DTH2242

Same as Cintiq 24HDT, DTH2242 has two interfaces sharing one configuration.
This patch ignores the second interface.

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