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