dax: Fix missing kdoc for dax_device

struct dax_device has a member named ops which was undocumented.

Add the kdoc.

Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20220304204655.3489216-1-ira.weiny@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>