staging: comedi: ni_at_ao: tidy up the calibration subdevice

The AT-AO-6/10 boards use DAC8800 TrimDACs to software calibrate the
analog output channels. These are exposed to the user as a calibration
subdevice.

Tidy up, and document, the calibration subdevice.

Since the TrimDACs are not readable, store the calibration values in the
private data for the user to read back as needed.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>