staging: comedi: pcmmio: move ao shadow data to device private data

There is only one ai subdevice in this driver so there is no reason
to hold the last sample written to each channel in the subdevice
private data. Move the data into the device private data,

This gets some of the data out of the subdevice private data union
and removes some of the uses of the ugly 'subpriv' macro.

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>