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

cros_ec_get_proto_info() expects to receive
sizeof(struct ec_response_get_protocol_info) 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().
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20220609084957.3684698-10-tzungbi@kernel.org

drivers/platform/chrome/cros_ec_proto.c

@@ -314,6 +314,11 @@ static int cros_ec_get_proto_info(struct cros_ec_device *ec_dev, int devidx)
 		goto exit;
 	}
 
+	if (ret == 0) {
+		ret = -EPROTO;
+		goto exit;
+	}
+
 	info = (struct ec_response_get_protocol_info *)msg->data;
 
 	switch (devidx) {