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/siemens/common/fpga.c b/board/siemens/common/fpga.c
index 5660c09..ef8bfde 100644
--- a/board/siemens/common/fpga.c
+++ b/board/siemens/common/fpga.c
@@ -286,8 +286,7 @@
     return 0;
 
  failure:
-    cmd_usage(cmdtp);
-    return 1;
+    return cmd_usage(cmdtp);
 }
 
 U_BOOT_CMD(