staging: comedi: cb_pcimdas: cleanup cb_pcimdas_ao_winsn()

The comedi core validates the insn->chanspec and data values before
calling the (*insn_write) functions. The 'chan' will always be valid
and the data values do not need to be masked.

Tidy up this function and remove the unnecessary code.

For aesthetics, rename the function.

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>