Input: wacom - use HID core to actually fetch the report descriptor

HID core already retrieves the report descritor. There is no need
to ask ourself for one.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Jason Gerecke <killertofu@gmail.com>
Tested-by: Jason Gerecke <killertofu@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>