acpi, nfit: fix health event notification

Integration testing with a BIOS that generates injected health event
notifications fails to communicate those events to userspace. The nfit
driver neglects to link the ACPI DIMM device with the necessary driver
data so acpi_nvdimm_notify() fails this lookup:

        nfit_mem = dev_get_drvdata(dev);
        if (nfit_mem && nfit_mem->flags_attr)
                sysfs_notify_dirent(nfit_mem->flags_attr);

Add the necessary linkage when installing the notification handler and
clean it up when the nfit driver instance is torn down.

Cc: <stable@vger.kernel.org>
Cc: Toshi Kani <toshi.kani@hpe.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Fixes: ba9c8dd3

 ("acpi, nfit: add dimm device notification support")
Reported-by: Daniel Osawa <daniel.k.osawa@intel.com>
Tested-by: Daniel Osawa <daniel.k.osawa@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>