serdev: fix receive_buf return value when no callback

The receive_buf callback is supposed to return the number of bytes
processed and should specifically not return a negative errno.

Due to missing sanity checks in the serdev tty-port controller, a driver
not providing a receive_buf callback could cause the flush_to_ldisc()
worker to spin in a tight loop when the tty buffer pointers are
incremented with -EINVAL (-22).

The missing sanity checks have now been added to the tty-port
controller, but let's fix up the serdev-controller helper as well.
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/serdev.h

@@ -184,7 +184,7 @@ static inline int serdev_controller_receive_buf(struct serdev_controller *ctrl,
 	struct serdev_device *serdev = ctrl->serdev;
 
 	if (!serdev || !serdev->ops->receive_buf)
-		return -EINVAL;
+		return 0;
 
 	return serdev->ops->receive_buf(serdev, data, count);
 }