mxssb: Clean up error messages

Use -EINVAL instead of -1 in the error messages.
Signed-off-by: Marek Vasut <marex@denx.de>

mxssb.c

@@ -1018,7 +1018,6 @@ static int sb_parse_image(struct sb_image_ctx *ictx)
 		return ret;
 
 	return 0;
-
 }
 
 static void sb_free_image(struct sb_image_ctx *ictx)
@@ -1085,24 +1084,24 @@ int main(int argc, char **argv)
 			break;
 		case 'h':
 			print_help(argv[0]);
-			return -1;
+			return -EINVAL;
 		default:
 			fprintf(stderr, "ERROR: Invalid option '%c'.\n\n", opt);
 			print_help(argv[0]);
-			return -1;
+			return -EINVAL;
 		}
 	}
 
 	if (optind != argc) {
 		fprintf(stderr, "ERROR: Expected argument after options!\n\n");
 		print_help(argv[0]);
-		return -1;
+		return -EINVAL;
 	}
 
 	if (!ctx.cfg_filename || !output_filename) {
 		fprintf(stderr, "ERROR: Input files not specified!\n\n");
 		print_help(argv[0]);
-		return -1;
+		return -EINVAL;
 	}
 
 	ret = sb_parse_image(&ctx);