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/common/cmd_mtdparts.c b/common/cmd_mtdparts.c
index 447486b..ceec5a9 100644
--- a/common/cmd_mtdparts.c
+++ b/common/cmd_mtdparts.c
@@ -1845,8 +1845,7 @@
 		return delete_partition(argv[2]);
 	}
 
-	cmd_usage(cmdtp);
-	return 1;
+	return cmd_usage(cmdtp);
 }
 
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