dax: super.c: fix kernel-doc bad line warning

Convert an empty line to " *" to avoid a kernel-doc warning:

drivers/dax/super.c:478: warning: bad line:
Signed-off-by: NRandy Dunlap <rdunlap@infradead.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: nvdimm@lists.linux.dev
Link: https://lore.kernel.org/r/20230117070249.31934-1-rdunlap@infradead.orgSigned-off-by: NDan Williams <dan.j.williams@intel.com>

drivers/dax/super.c

@@ -475,7 +475,7 @@ EXPORT_SYMBOL_GPL(put_dax);
 /**
  * dax_holder() - obtain the holder of a dax device
  * @dax_dev: a dax_device instance
-
+ *
  * Return: the holder's data which represents the holder if registered,
  * otherwize NULL.
  */