env: Fix return values in env_attr_lookup()

This function returned numbers for error codes. Change them to error
codes.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/common/env_attr.c b/common/env_attr.c
index 64baca5..e791f44 100644
--- a/common/env_attr.c
+++ b/common/env_attr.c
@@ -148,10 +148,10 @@
 
 	if (!attributes)
 		/* bad parameter */
-		return -1;
+		return -EINVAL;
 	if (!attr_list)
 		/* list not found */
-		return 1;
+		return -EINVAL;
 
 	entry = reverse_strstr(attr_list, name, NULL);
 	while (entry != NULL) {
@@ -209,5 +209,5 @@
 	}
 
 	/* not found in list */
-	return 2;
+	return -ENOENT;
 }