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>