staging: comedi: das08: Rearrange AO write functions

Combine the AO write functions for 'JR' boards and other boards into a
single function and factor out the data writing from the comedi
instruction handling so it can be called from elsewhere in a later
patch.

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