staging: comedi: quatech_daqp_cs: introduce daqp_ai_get_sample()

Introduce a helper function to get a two's complement sample from
the FIFO and munge it to the offset binary format that comedi uses.

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>