staging: comedi: das08: Support read-back of AO subdevice

Stash the last value written to each AO channel in private data and
support the INSN_READ instruction to read it back.  Don't bother setting
the SDF_READABLE subdevice flag though as the hardware isn't really
readable - we're just faking it.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>