default or selected kernelversion does not have config value CONFIG_AD799X_RING_BUFFER.
Result is shown for kernelversion 6.5.1

Analog Devices AD799x: use ring buffer


Linux Kernel Configuration
└─>Device Drivers
└─>Staging drivers
└─>IIO staging drivers
└─>Analog to digital converters
└─>Analog Devices AD799x: use ring buffer
In linux kernel since version 2.6.37 (release Date: 2011-01-04)  
Say yes here to include ring buffer support in the AD799X
ADC driver.