staging:iio:ad7780: Use common Sigma Delta library

Convert the ad7780 driver to make use of the new common code for devices from
the Analog Devices Sigma Delta family.

As a bonus the ad7780 driver gains support for buffered mode. Although this is a
bit tricky. The ad7780 reports in the lower 4 unused bits of the data word the
internal gain used. The driver will update the scale attribute value depending
on the gain accordingly, but obviously this will only work if the gain does not
change while sampling. This is not perfect, but since we store the raw value in
the buffer an application which is aware of this can extract the gain factor
from the buffer as well an apply it accordingly.

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