staging:iio:adc:ad7152 bring more into line with abi.

Get rid of explicit writes to registers and conversion mode
control.

Couple of bits I'm unsure about.
* Do calibration modes self reset when done?  How do you tell they are
done?

* Should we poll the status register just to be sure we have a new conversion?

All done sans hardware.

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