cbfs: Fix typo in cbfs_prog_stage_load()

The proper return value to signal an error from cbfs_prog_stage_load()
is -1, not 0.

Change-Id: Ie53b0359c7c036e3f809d1f941dab53f090b84ab
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/12633
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins)

src/lib/cbfs.c

@@ -202,7 +202,7 @@ int cbfs_prog_stage_load(struct prog *pstage)
 	const struct region_device *fh = prog_rdev(pstage);
 
 	if (rdev_readat(fh, &stage, 0, sizeof(stage)) != sizeof(stage))
-		return 0;
+		return -1;
 
 	fsize = region_device_sz(fh);
 	fsize -= sizeof(stage);