Bluetooth: btmtkuart: Use serdev_device_write_buf() instead of serdev_device_write()

serdev_device_write() is not appropriate at here because
serdev_device_write_wakeup() is not used to release completion hold
by the former at @write_wakeup member of struct serdev_device_ops.

Fix by using serdev_device_write_buf() instead of serdev_device_write().
Signed-off-by: Zijun Hu <zijuhu@codeaurora.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

drivers/bluetooth/btmtkuart.c

@@ -695,8 +695,7 @@ static int btmtkuart_change_baudrate(struct hci_dev *hdev)
 
 	/* Send a dummy byte 0xff to activate the new baudrate */
 	param = 0xff;
-	err = serdev_device_write(bdev->serdev, &param, sizeof(param),
-				  MAX_SCHEDULE_TIMEOUT);
+	err = serdev_device_write_buf(bdev->serdev, &param, sizeof(param));
 	if (err < 0 || err < sizeof(param))
 		return err;