staging: comedi: dmm32at: cleanup dmm32at_ao_winsn()

For aesthetics, rename the function and tidy it up a bit. Also rename the
registers used by this function and add a macro to set the 'chan' bits in
the MSB data register.

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>