libnvdimm, nfit: Use ACPI_SIG_NFIT instead of hard coded string

It's minor but that's still better to use ACPI_SIG_NFIT instead of hard
coded string.

Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Len Brown <lenb@kernel.org>
Signed-off-by: Lee, Chun-Yi <jlee@suse.com>
Acked-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>