The pdev->dev.of_node can be NULL if the "serial" node is absent.
Add a NULL check to return an error in such cases.

Fixes: 930cbf92 ("tty: serial: Add Nuvoton ma35d1 serial driver support")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/all/8df7ce45-fd58-4235-88f7-43fe7cd67e8f@moroto.mountain/Signed-off-by: Jacky Huang <ychuang3@nuvoton.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Cc: stable <stable@kernel.org>
Link: https://lore.kernel.org/r/20240625064128.127-1-ychuang570808@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>