driver core: Document struct device:dma_ops

Commit 5657933d (treewide: Move dma_ops from struct dev_archdata into
struct device) added the dma_ops field to struct device, but did not
update the kerneldoc comment, yielding this warning:

  ./include/linux/device.h:969: warning: No description found for parameter 'dma_ops'

Add a description and bring a little peace to the world.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/device.h

@@ -847,6 +847,7 @@ struct dev_links_info {
  * @msi_list:	Hosts MSI descriptors
  * @msi_domain: The generic MSI domain this device is using.
  * @numa_node:	NUMA node this device is close to.
+ * @dma_ops:    DMA mapping operations for this device.
  * @dma_mask:	Dma mask (if dma'ble device).
  * @coherent_dma_mask: Like dma_mask, but for alloc_coherent mapping as not all
  * 		hardware supports 64-bit addresses for consistent allocations