ieee1275: Fix crash in of_path_of_nvme when of_path is empty

ieee1275: Fix crash in of_path_of_nvme when of_path is empty

The of_path_of_nvme function (commit 2391d57, ieee1275: add nvme
support within ofpath) introduced a functional regression:

On systems which are not based on Open Firmware but have at
least one NVME device, find_obppath will return NULL and thus
trying to append the disk name to of_path will result in a
crash.

The proper behavior of of_path_of_nvme is, however, to just
return NULL in such cases, like other users of find_obppath,
such as of_path_of_scsi.

Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Reviewed-by: Eric Snowberg <eric.snowberg@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>