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/main.c b/common/main.c
index 20090ee..54ef79e 100644
--- a/common/main.c
+++ b/common/main.c
@@ -1418,10 +1418,8 @@
 {
 	int i;
 
-	if (argc < 2) {
-		cmd_usage(cmdtp);
-		return 1;
-	}
+	if (argc < 2)
+		return cmd_usage(cmdtp);
 
 	for (i=1; i<argc; ++i) {
 		char *arg;