staging: comedi: me4000: remove ao context

The ao context is a struct containing all the register addresses
used with the ao subdevice. These can easily be calculated when
needed. Remove the me4000_ao_context struct, its initialization
function, and it's variable in the private data.

The last value written to the ao channels still needs to be saved
in the private data for readback.

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