serial: Clear UPF_DEAD before calling tty_port_register_device_attr_serdev()

serial: Clear UPF_DEAD before calling tty_port_register_device_attr_serdev()

If a serdev_device_driver is already loaded for a serdev_tty_port when it
gets registered by tty_port_register_device_attr_serdev() then that
driver's probe() method will be called immediately.

The serdev_device_driver's probe() method should then be able to call
serdev_device_open() successfully, but because UPF_DEAD is still dead
serdev_device_open() will fail with -ENXIO in this scenario:

  serdev_device_open()
  ctrl->ops->open()	/* this callback being ttyport_open() */
  tty->ops->open()	/* this callback being uart_open() */
  tty_port_open()
  port->ops->activate()	/* this callback being uart_port_activate() */
  Find bit UPF_DEAD is set in uport->flags and fail with errno -ENXIO.

Fix this be clearing UPF_DEAD before tty_port_register_device_attr_serdev()
note this only moves up the UPD_DEAD clearing a small bit, before:

  tty_port_register_device_attr_serdev();
  mutex_unlock(&tty_port.mutex);
  uart_port.flags &= ~UPF_DEAD;
  mutex_unlock(&port_mutex);

after:

  uart_port.flags &= ~UPF_DEAD;
  tty_port_register_device_attr_serdev();
  mutex_unlock(&tty_port.mutex);
  mutex_unlock(&port_mutex);

Reported-by: Weifeng Liu <weifeng.liu.z@gmail.com>
Closes: https://lore.kernel.org/platform-driver-x86/20240505130800.2546640-1-weifeng.liu.z@gmail.com/
Tested-by: Weifeng Liu <weifeng.liu.z@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20240509141549.63704-1-hdegoede@redhat.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>