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>