staging: comedi: mpc624: introduce mpc624_ai_get_sample()

Introduce a helper function to read the analog sample from the serially
connected A/D converter and handle the munging of the data.

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>