Standardize command usage messages with cmd_usage()

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
diff --git a/common/cmd_sf.c b/common/cmd_sf.c
index 8c0a751..6dc522d 100644
--- a/common/cmd_sf.c
+++ b/common/cmd_sf.c
@@ -175,7 +175,7 @@
 		return do_spi_flash_erase(argc - 1, argv + 1);
 
 usage:
-	printf("Usage:\n%s\n", cmdtp->usage);
+	cmd_usage(cmdtp);
 	return 1;
 }