Input: wacom - fix error path in wacom_probe()

If we fail to retrieve HID descriptor we need to free allocated URB so
jump to proper label to do that.

Signed-off-by: Alexander Strakh <strakh@ispras.ru>
Acked-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>