default or selected kernelversion does not have config value CONFIG_FIXED_MII_100_FDX.
Result is shown for kernelversion 5.15.113

Emulation for 100M Fdx fixed PHY behavior

configname: CONFIG_FIXED_MII_100_FDX

Linux Kernel Configuration
└─>Device Drivers
└─>Network device support
└─>PHY device support
└─>Emulation for 100M Fdx fixed PHY behavior
In linux kernel since version 2.6.20 (release Date: 2007-02-04)