staging: comedi: c6xdigio: use comedi_offset_munge()

The value read from the encoders is in two's complement format. Use
the comedi_offset_munge() helper to convert the value to offset binary.

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>