staging: comedi: ni_labpc_common: introduce labpc_ao_write()

Introduce a helper function to write a value to a given DAC channel.

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>