staging: comedi: multiq3: tidy up multiq3_ai_insn_read()

For aesthetics, use the proper symbol when reading the A/D data register
to get the 16-bit sample data.

Use the comedi_offset_munge() to do the 2's complement to offset binary
munging of the sample data.

Tidy up the function a bit.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>