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/lwmon/lwmon.c b/board/lwmon/lwmon.c
index 61a1e14..9d6c21f 100644
--- a/board/lwmon/lwmon.c
+++ b/board/lwmon/lwmon.c
@@ -845,8 +845,7 @@
 	default:
 		break;
 	}
-	cmd_usage(cmdtp);
-	return 1;
+	return cmd_usage(cmdtp);
 }
 U_BOOT_CMD(
 	pic,	4,	1,	do_pic,
@@ -975,8 +974,7 @@
 	default:
 		break;
 	}
-	cmd_usage(cmdtp);
-	return 1;
+	return cmd_usage(cmdtp);
 }
 
 U_BOOT_CMD(