staging: iio: adc: Replace, rewrite ad7745 from scratch.

The existing ad7745 driver didn't conform with the IIO spec for such devices.
It was way simpler to rewrite the existing driver, than actually fixing it.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>