default or selected kernelversion does not have config value CONFIG_SX.
Result is shown for kernelversion 5.6.18

Specialix SX (and SI) card support

modulename: sx.ko

configname: CONFIG_SX

Linux Kernel Configuration
└─>Device Drivers
└─>Staging drivers
└─>Specialix SX (and SI) card support
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  
This is a driver for the SX and SI multiport serial cards.
Please read the file Documentation/serial/sx.txt for details.

This driver can only be built as a module ( = code which can be
inserted in and removed from the running kernel whenever you want).
The module will be called sx. If you want to do that, say M here.

source code: