staging: comedi: pcmmio: cleanup ao_winsn()

Rename this function to have namespace associated with the driver.

Refactor the function to remove the extra write to the command register
to set the range before writing the DAC value. Since the range is constant
for the entire comedi_insn it only needs to be set once. All writes to the
DAC after that will use the same range.

Define the register map for the analog output registers.

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>