drm/nouveau/core: refactor deprecated strncpy

`strncpy` is deprecated for use on NUL-terminated destination strings [1].

We should prefer more robust and less ambiguous string interfaces.

A suitable replacement is `strscpy` [2] due to the fact that it guarantees
NUL-termination on the destination buffer without unnecessarily NUL-padding.

There is likely no bug in the current implementation due to the safeguard:
| 	cname[sizeof(cname) - 1] = '\0';
... however we can provide simpler and easier to understand code using
the newer (and recommended) `strscpy` api.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230914-strncpy-drivers-gpu-drm-nouveau-nvkm-core-firmware-c-v1-1-3aeae46c032f@google.com

drivers/gpu/drm/nouveau/nvkm/core/firmware.c

@@ -79,8 +79,7 @@ nvkm_firmware_get(const struct nvkm_subdev *subdev, const char *fwname, int ver,
 	int i;
 
 	/* Convert device name to lowercase */
-	strncpy(cname, device->chip->name, sizeof(cname));
-	cname[sizeof(cname) - 1] = '\0';
+	strscpy(cname, device->chip->name, sizeof(cname));
 	i = strlen(cname);
 	while (i) {
 		--i;