I can't see a download link for your AD7091R-5 driver for Linux anywhere. I looked here: AD7091R5 ADC Linux Driver [Analog Devices Wiki]
Can someone please provide a link?
It appears the AD7091R-5 Linux driver may never have been completed. Here is the link to another I2C SAR ADC that operates very similarly. This can be used as a template to start your project:
AD7291 IIO ADC Linux Driver [Analog Devices Wiki]