cmd_usage(): simplify return code handling

Lots of code use this construct:

	cmd_usage(cmdtp);
	return 1;

Change cmd_usage() let it return 1 - then we can replace all these
ocurrances by

	return cmd_usage(cmdtp);

This fixes a few places with incorrect return code handling, too.

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/board/amcc/luan/luan.c b/board/amcc/luan/luan.c
index c0368c0..c09d730 100644
--- a/board/amcc/luan/luan.c
+++ b/board/amcc/luan/luan.c
@@ -223,8 +223,7 @@
 			l2cache_status() ? "ON" : "OFF");
 		return 0;
 	default:
-		cmd_usage(cmdtp);
-		return 1;
+		return cmd_usage(cmdtp);
 	}
 
 	return  0;