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/pdm360ng/pdm360ng.c b/board/pdm360ng/pdm360ng.c
index 29a095d..e3abeb8 100644
--- a/board/pdm360ng/pdm360ng.c
+++ b/board/pdm360ng/pdm360ng.c
@@ -635,10 +635,8 @@
 static int cmd_lcd_brightness(cmd_tbl_t *cmdtp, int flag,
 			      int argc, char * const argv[])
 {
-	if (argc < 2) {
-		cmd_usage(cmdtp);
-		return 1;
-	}
+	if (argc < 2)
+		return cmd_usage(cmdtp);
 
 	return set_lcd_brightness(argv[1]);
 }