platform/chrome: cros_ec_uart: fix negative type promoted to high

serdev_device_write_buf() returns negative numbers on errors.  When
the return value compares to unsigned integer `len`, it promotes to
quite large positive number.

Fix it.

Fixes: 04a8bdd1 ("platform/chrome: cros_ec_uart: Add transport layer")
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Benson Leung <bleung@chromium.org>
Link: https://lore.kernel.org/r/20230109081554.3792547-1-tzungbi@kernel.org

drivers/platform/chrome/cros_ec_uart.c

@@ -149,9 +149,10 @@ static int cros_ec_uart_pkt_xfer(struct cros_ec_device *ec_dev,
 	resp->status = 0;
 
 	ret = serdev_device_write_buf(serdev, ec_dev->dout, len);
-	if (ret < len) {
+	if (ret < 0 || ret < len) {
 		dev_err(ec_dev->dev, "Unable to write data\n");
-		ret = -EIO;
+		if (ret >= 0)
+			ret = -EIO;
 		goto exit;
 	}