staging: comedi: das16: introduce das16_ai_{en, dis}able() helpers

Introduce a couple helper functions to enable and disable the
analog input interrupt/dma conversions.

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>