Input: wacom - properly enable runtime PM

We need to always call usb_autopm_put_interface() in wacom_open(),
not only when initialization fails, otherwise the device will be
marked as PM-busy and will never be put in suspended state.

Based on patch by Oliver Neukum.

Acked-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>