efi_loader: using EFI_UNSUPPORTED for private authenticated variables

Improve error message for UEFI SCT tests.

Signed-off-by: Weizhao Ouyang <o451686892@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/lib/efi_loader/efi_variable.c b/lib/efi_loader/efi_variable.c
index 2951dc7..e6c1219 100644
--- a/lib/efi_loader/efi_variable.c
+++ b/lib/efi_loader/efi_variable.c
@@ -163,6 +163,7 @@
 		break;
 	default:
 		/* TODO: support private authenticated variables */
+		ret = EFI_UNSUPPORTED;
 		goto err;
 	}