linux-phytec-fsl

Documentation: rs485: Do not define manually the ioctl

It is not a very good practice to define the IOCTL manually instead of
using the header file.

Fix it on the documentation example.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>