staging:iio:ad7780: Add support for the ad7170/ad7171

The ad7170/ad7171 have a software interface similar to the ad7780. They do not
have an external pin which allows to change the internal gain and the what is
used for the gain bit in the ad7780/ad7781 becomes part of the check pattern.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>