staging: comedi: adl_pci6208: cleanup pci6208_ao_winsn()

staging: comedi: adl_pci6208: cleanup pci6208_ao_winsn()

Use the defines for the register map to make the code a bit
clearer. Shorten or rename some of the local variables for
the same reason.

Only the last value written to the DAC needs to be cached in
the ao_readback variable.

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>