USB: Gadget: Webcam: Return correct result of bind

The config bind was reported success even if usb_add_function
failed. Fix the return value.

Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>