mflash: use common cmd_usage function

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/common/cmd_mgdisk.c b/common/cmd_mgdisk.c
index c2d5217..51c994b 100644
--- a/common/cmd_mgdisk.c
+++ b/common/cmd_mgdisk.c
@@ -54,8 +54,7 @@
 			return 1;
 		break;
 	default:
-		printf("Usage:\n%s\n", cmdtp->usage);
-		return 1;
+		return cmd_usage(cmdtp);
 	}
 	return 0;
 }