staging: comedi: das1800: introduce das1800_ai_chanspec_bits()

Introduce a helper function for the common code needed to set the
control c register bits for a channel specification.

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>