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