iio: ad799x: Fix ad7991/ad7995/ad7999 config setup

The ad7991/ad7995/ad7999 does not have a configuration register like the
other devices that can be written and read. The configuration is written as
part of the conversion sequence.

Fixes: 0f7ddcc1 ("iio:adc:ad799x: Write default config on probe and reset alert status on probe")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Tested-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>