vbe: Use a warning for a failed requests

Optional requests should present a warning rather than an error. Update
the log call.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/vbe_request.c b/boot/vbe_request.c
index fa01892..312edfa 100644
--- a/boot/vbe_request.c
+++ b/boot/vbe_request.c
@@ -195,8 +195,8 @@
 		ret = vbe_process_request(dest, &result);
 		if (ret) {
 			result.errnum = ret;
-			log_err("Failed to process VBE request %s (err=%d)\n",
-				ofnode_get_name(dest), ret);
+			log_warning("Failed to process VBE request %s (err=%d)\n",
+				    ofnode_get_name(dest), ret);
 			if (*result.err_str) {
 				char *msg = strdup(result.err_str);