Input: wacom - fix "can not retrieve extra class descriptor" for 24HDT

The MFT device in the Cintiq 24HDT has two interfaces sharing the
same configuration. Without this patch, the driver attempts to
make use of both interfaces, even though the second interface is
not compatible with this driver.

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