Standardize command usage messages with cmd_usage()

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
diff --git a/board/ads5121/ads5121_diu.c b/board/ads5121/ads5121_diu.c
index 896900b..11e2be7 100644
--- a/board/ads5121/ads5121_diu.c
+++ b/board/ads5121/ads5121_diu.c
@@ -79,7 +79,7 @@
 	unsigned int addr;
 
 	if (argc < 2) {
-		printf("Usage:\n%s\n", cmdtp->usage);
+		cmd_usage(cmdtp);
 		return 1;
 	}