kernelconfig_penguin
default or selected kernelversion does not have config value CONFIG_AD799X_RING_BUFFER.
Result is shown for kernelversion 3.9.9

Analog Devices AD799x: use ring buffer

configname: CONFIG_AD799X_RING_BUFFER

Linux Kernel Configuration
└─> Device Drivers
└─> Staging drivers
└─> IIO staging drivers
└─> Analog to digital converters
└─> Analog Devices AD799x: use ring buffer

Say yes here to include ring buffer support in the AD799X
ADC driver.