mflash: use common cmd_usage function

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

common/cmd_mgdisk.c

@@ -54,8 +54,7 @@ int do_mg_disk_cmd (cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 			return 1;
 		break;
 	default:
-		printf("Usage:\n%s\n", cmdtp->usage);
-		return 1;
+		return cmd_usage(cmdtp);
 	}
 	return 0;
 }