dma-buf/dma-fence: improve doc for dma_fence_add_callback()

Document return values for this function.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170215175725.6692-1-gustavo@padovan.org

drivers/dma-buf/dma-fence.c

@@ -239,6 +239,8 @@ EXPORT_SYMBOL(dma_fence_enable_sw_signaling);
  * after it signals with dma_fence_signal. The callback itself can be called
  * from irq context.
  *
+ * Returns 0 in case of success, -ENOENT if the fence is already signaled
+ * and -EINVAL in case of error.
  */
 int dma_fence_add_callback(struct dma_fence *fence, struct dma_fence_cb *cb,
 			   dma_fence_func_t func)