dma-buf: Fix kerneldoc comment for struct dma_fence_array

The kerneldoc comment for struct dma_fence_array lacks a description
of the "work" member, leading to this docs-build warning:

./include/linux/dma-fence-array.h:54: warning: Function parameter or member 'work' not described in 'dma_fence_array'

Add a description and make the warning go away.

Signed-off-by: Jonathan Corbet <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

include/linux/dma-fence-array.h

···
  * @num_fences: number of fences in the array
  * @num_pending: fences in the array still pending
  * @fences: array of the fences
+ * @work: internal irq_work function
  */
 struct dma_fence_array {
 	struct dma_fence base;