platform/chrome: cros_ec_proto: handle empty payload in getting info legacy

cros_ec_get_proto_info_legacy() expects to receive
sizeof(struct ec_response_hello) from send_command().  The payload is
valid only if the return value is positive.

Return -EPROTO if send_command() returns 0 in
cros_ec_get_proto_info_legacy().
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20220609084957.3684698-13-tzungbi@kernel.org

drivers/platform/chrome/cros_ec_proto.c

@@ -356,7 +356,7 @@ static int cros_ec_get_proto_info_legacy(struct cros_ec_device *ec_dev)
 	struct cros_ec_command *msg;
 	struct ec_params_hello *params;
 	struct ec_response_hello *response;
-	int ret;
+	int ret, mapped;
 
 	ec_dev->proto_version = 2;
 
@@ -377,12 +377,18 @@ static int cros_ec_get_proto_info_legacy(struct cros_ec_device *ec_dev)
 		goto exit;
 	}
 
-	ret = cros_ec_map_error(msg->result);
-	if (ret) {
+	mapped = cros_ec_map_error(msg->result);
+	if (mapped) {
+		ret = mapped;
 		dev_err(ec_dev->dev, "EC responded to v2 hello with error: %d\n", msg->result);
 		goto exit;
 	}
 
+	if (ret == 0) {
+		ret = -EPROTO;
+		goto exit;
+	}
+
 	response = (struct ec_response_hello *)msg->data;
 	if (response->out_data != 0xa1b2c3d4) {
 		dev_err(ec_dev->dev,