staging: iio: adc: ad7192: disable burnout currents on misconfig

The burnout currents can be enabled only if buffer is enabled
and CHOP is disabled.

So, if neither of these conditions are met, then
we should disable the burnout currents in the driver as well,
and warn the user.

This change doesn't fix anything.
The burnout currents simply won't work if CHOP is enabled
or buffer is disabled.
The intent is to provide the user with some feedback
instead of silently not working inside the chip.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>