tty: serdev: drop serdev_controller_ops::write_room()

In particular, serdev_device_write_room() is not called, so the whole
serdev's write_room() can go.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20250317070046.24386-17-jirislaby@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>