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_setexpr.c b/common/cmd_setexpr.c
index b3e489f..1ff1232 100644
--- a/common/cmd_setexpr.c
+++ b/common/cmd_setexpr.c
@@ -57,10 +57,8 @@
 	int w;
 
 	/* Validate arguments */
-	if ((argc != 5) || (strlen(argv[3]) != 1)) {
-		cmd_usage(cmdtp);
-		return 1;
-	}
+	if ((argc != 5) || (strlen(argv[3]) != 1))
+		return cmd_usage(cmdtp);
 
 	w = cmd_get_data_size(argv[0], 4);