dax: Use device_unregister() in unregister_dax_mapping()

Replace an open-coded device_unregister() sequence with the helper.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/r/168577283989.1672036.7777592498865470652.stgit@dwillia2-xfh.jf.intel.com


Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Fan Ni <fan.ni@samsung.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>