env: Replace all open-coded gd->env_valid values with ENV_ flags

Some of these were missed in the conversion.
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/env/nand.c b/env/nand.c
index dea7b00..8058b55 100644
--- a/env/nand.c
+++ b/env/nand.c
@@ -79,7 +79,7 @@
 
 	if (!crc1_ok && !crc2_ok) {
 		gd->env_addr	= 0;
-		gd->env_valid	= 0;
+		gd->env_valid	= ENV_INVALID;
 
 		return 0;
 	} else if (crc1_ok && !crc2_ok) {