Docs: iio: ad7191 Correct clock configuration

Correct the ad7191 documentation to match the datasheet:
- Fix inverted CLKSEL pin logic: device uses external clock when pin is
  inactive, and internal CMOS/crystal when high.
- Correct CMOS-compatible clock pin from MCLK2 to MCLK1.

Signed-off-by: Ammar Mustafa <ammarmustafa34@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>