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: Randy 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.org
Signed-off-by: Dan Williams <dan.j.williams@intel.com>