ieee1275: fix build regression in of_path_of_nvme

ieee1275: fix build regression in of_path_of_nvme

The of_path_of_nvme function (commit 2391d57, ieee1275: add nvme
support within ofpath) introduced a build regression:
    grub-core/osdep/linux/ofpath.c:365:21: error: comparison between pointer
    and zero character constant [-Werror=pointer-compare]
       if ((digit_string != '\0') && (*part_end == 'p'))

Update digit_string to compare against the char instead of the pointer.

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>